India Asia Continental Collision Research Articles

Carbon cycling during the India-Asia collision revealed by δ26Mg−δ66Zn−δ98Mo evidence from ultrapotassic volcanoes in NW Tibet

India-Asia continental collision−induced volcanic gas emissions are thought to have played an important role in driving Cenozoic atmospheric CO2 variations, yet the details of how the deep carbon cycle may influence volcanic CO2 degassing are not understood. We present a novel study employing Mg-Zn-Mo isotopic compositions of Cenozoic ultrapotassic lavas from NW Tibet. The negative Mg-Zn isotope correlation (δ26Mg = −0.39‰ to −0.19‰; δ66Zn = +0.27‰ to +0.36‰), bolstered by petrographic analysis of mantle-derived xenoliths from these lavas, demonstrates that the ultrapotassic magmas originated from a lithospheric mantle source that had been enriched by recycled carbonate-bearing sediments rich in calcite and dolomite. Very low δ98Mo values (−0.78‰ to 0‰) relative to the average continental crust (δ98Mo = +0.10‰ to +0.35‰) further indicate that the sedimentary components were derived from the subducted Indian continental crust after its dehydration. Monte Carlo modeling estimates that the input flux of carbon (elemental C) from such sediments into the lithospheric mantle is ∼5.6 Mt/yr, with a predicted CO2 emission rate of ∼15.5 Mt/yr. We suggest that the still ongoing subduction of the Indian tectonic plate has played a crucial role in introducing substantial quantities of carbonate-rich sediments into the Tibetan lithospheric mantle, leading to the sequestration of large amounts of CO2 via carbonatite metasomatism. Hence, partial melting of such a carbon-rich mantle reservoir in an orogenic setting provides the positive feedback mechanism that can explain the high flux of volcanic CO2 during India-Asia collision. These findings not only highlight the importance of continental subduction, sediment recycling, and mantle metasomatism by carbon-rich melts/fluids in the generation of Tibetan ultrapotassic volcanism, but they also show how the deep carbon cycle influences volcanic CO2 degassing.

A high-resolution seismic velocity model for East Asia using full-waveform tomography: Constraints on India-Asia collisional tectonics

East Asia's complex tectonic history makes it a unique place for plate-tectonic studies. While the crust and mantle structures beneath this region have been extensively investigated by receiver function and many different kinds of tomography studies, discrepancies among seismic velocity models have complicated our understanding of East Asia's tectonic evolution. Here, we use full-waveform adjoint tomography to study the seismic P- and S-wave velocity structures of the crust and mantle beneath East Asia. We use a large waveform dataset from 141 earthquakes recorded by 4640 broadband seismic stations. Our inversion optimizes the normalized correlation coefficient between synthetic and observed seismograms within individual time windows of different seismic phases, from P arrivals to the slowest surface waves. This approach allows us to fit regional multipathed waveforms and provides very well-resolved seismic P- and S-wave velocity structures from the crust to depths of about 800 km. Furthermore, we are able to obtain this resolution over most of East Asia allowing us to compare the amplitude and shapes of anomalies across the entire region. The observed structures in our new full-waveform tomography model (FWEA23) correlate well with tectonic units, revealing sharp contrasts across tectonic boundaries. Our model reveals narrow and strong linear fast anomalies associated with subduction zones in the western Pacific and Southeast Asia. Compared to these oceanic slabs, we observe different and more complicated mantle structures beneath the India-Asia continental collision zone. While our model spans a wide region in East Asia, we focus our interpretations on the seismic velocity anomalies beneath Tibet and surrounding regions, as our model provides significant implications for India-Asia continental collision tectonics. Our analysis suggests that cratonic Indian lithosphere collided with Eurasian plate around 25–20 Ma, subsequently underthrusting beneath Tibet horizontally with little deformation. This resulted in thickening, destabilization, and delamination of the rheologically weaker and less buoyant mantle lithosphere beneath north-central Tibet. Moreover, our model reveals isolated lithospheric fragments within the mantle transition zone and lower mantle beneath southern Tibet and northern India, suggesting that non-cratonic Greater India continental lithosphere has broken off and sunk into the deeper mantle. The lack of continuity among these lithospheric fragments suggests a convective destabilization process, distinct from typical oceanic plate subduction. Our results emphasize the important role of rheological properties of continental lithospheres in shaping the dynamics of continental collision.

Pressure-temperature-time evolution of the Milin pelitic schists: Implications for the tectonic evolution of the eastern Indus-Yarlung suture zone, eastern Himalaya

We report an integrated comprehensive dataset composed of petrography, pressure-temperature (P-T) calculations, monazite U-Th-Pb ages, and trace-element data from pelitic schists in the eastern Indus-Yarlung suture zone in the Milin area of the eastern Himalaya. These rocks represent the exposure of subduction-related rocks within the eastern Indus-Yarlung suture zone accretionary complex. The dominant mineral assemblages of the pelitic schists are garnet + kyanite + staurolite + biotite + quartz and garnet + kyanite + staurolite + biotite + paragonite + sillimanite with quartz, plagioclase, and ilmenite assemblages. Phase equilibrium modeling of sillimanite-bearing pelitic schists yielded peak P-T conditions of ∼670−680 °C at ∼8.6 kbar, similar to that of kyanite-bearing schists (∼670 °C, ∼8.8 kbar). Monazite grains with complex internal structures retained variable ages ranging from 28 Ma to 15 Ma, which correlate systematically with changes in the concentrations of Y, Th, U, and heavy rare earth elements and ratios of Th/U. Combined with petrologic analysis, we conclude that the pelitic schists experienced a long-lived prograde metamorphism from ca. 28 Ma to ca. 22 Ma. Peak metamorphism occurred in the period 22−21 Ma, followed by quasi-isothermal decompression until 15 Ma. The discrepancies among metamorphic P-T-t paths in the eastern Indus-Yarlung suture zone indicate the presence of not only collision-related regional metamorphism at medium P-T conditions, but also subduction-related high-pressure−low-temperature terranes in the Milin region. These two domains experienced different P-T evolution and tectonic histories and were juxtaposed in the early Neogene during the India-Asia continental collision.

Degassing of Mantle‐Derived Helium From Hot Springs Along the India‐Asia Continental Collision Settings: Origins, Migration Velocity and Flux

AbstractMantle‐derived volatile degassing lacks quantitative evaluation in continental regions without active magmatism, such as the Tibetan Plateau. Ten new gas abundance and helium isotope data points combined with 286 hydrothermal volatile literature data points in India–Asia continental collision settings demonstrate widespread mantle‐derived volatiles across the thick (∼70 km) crust. The mantle‐derived 3He is best explained by direct mantle volatile input from subcontinental lithospheric mantle (1%–36.5%) or the asthenospheric wedge (1%–27.8%) instead of fossil residual magmatic fluids or Quaternary mantle‐derived melt intrusion into crustal depth. Mantle‐derived 3He is transported from the deep mantle to the surface at an upflow rate of 30–11,700 mm/year based on a newly developed steady‐state one‐dimensional flow model, corresponding to a mantle‐derived 3He flux of 17 to 1.5 × 107 atoms m−2 s−1 (81.7%–99.4% of the total 3He flux) and a mantle‐derived 4He flux from 2.0 × 106 to 1.8 × 1012 atoms m−2 s−1 (1.4%–36.5% of the total 4He flux). The mantle‐derived helium fluxes in the study area are comparable to those of other nonvolcanic hydrothermal systems in the tectonically active regions but lower than those of volcanic fields. The helium transit time ranges from 5.3 ka to 2.3 Ma, indicating that the spatial pattern of 3He/4He ratios in the India‐Asia continental collision settings can provide a snapshot of the state of the Indian mantle lithosphere between those revealed by potassic‐rich mafic rocks (25–8 Ma) and seismic methods (present).

Constraints on geological structures and dynamics along the Tian Shan-Pamir orogenic belt with helium isotopes in hot springs

Located in the western part of the India-Asia continental collision zone, the Tian Shan-Pamir range is the currently most active and largest intracontinental orogenic belt worldwide. The uplifting process of the Pamir Plateau and deep structures below the Tian Shan orogen has attracted a lot of attention. The behavior of He in the geothermal systems holds great potential for addressing questions about local deep crustal processes and the Plateau uplifting mechanism. However, there is still a lack of study on volatiles in the Tian Shan-Pamir region, especially, helium, which is a very powerful geochemistry tool to reflect deep structures and processes. Through constraining the fluxes of 3He, we recognized different transportation modes for deep volatiles between Tian Shan and Pamir. The mantle-derived He along the Tian Shan is connected to the main faults at the mountain foot of the North Tian Shan and the South Tian Shan, and the rapid ascent of fluid from the mantle is implied. The relatively high levels of 3He/4He on the Pamir Plateau are indicative of crustal magmatism that is younger than ∼11 Ma, indicating a contribution of mantle-derived melts to the crustal thickening and uplifting of the Pamir Plateau since the middle Miocene.

Geochronology and ore-forming fluid source of the Buzhu orogenic gold deposit in southern Tibet

The Buzhu gold deposit is a newly discovered and representative gold deposit in the Indus-Yarlung Zangbo suture zone. At Buzhu, auriferous quartz veins with paragenetic sequences of quartz-pyrite-sericite hosted by Early Jurassic greenschist facies rocks, and gold occurs mainly as invisible lattice gold in pyrite as well as minor as visible micrograins inside (arseno)pyrite fractures. We have recognized two auriferous pyrite generations formed in the syn-ore stage (Py2a and Py2b). Py2a is euhedral-subhedral granular with cavities, whilst Py2b mainly grew colloidally around Py2a, indicating that pre-existing pyrite may have been dissolved to form the porous Py2a and Py2b rim. Py2a and Py2b have similar δ34SV-CDT values of 6.98‰ to 7.54‰ and 6.18‰ to 6.85‰, respectively, which are also similar to those of the ore-hosting sediments (6.08‰ to 7.08‰). In-situ trace element analysis shows that Py2b (avg. 1.63% Au) contains more gold than Py2a (avg. 0.42% Au). Combined with petrological and geochemical data, we hypothesized that the gold was derived from the metamorphic devolatilization of the sedimentary wallrocks, and pyrite dissolution-reprecipitation has driven the gold enrichment from Py2a to Py2b. The sericite Ar-Ar date of 19.60 ± 0.66 Ma likely represents the gold mineralization age, as the dated sericite coexists with the auriferous pyrite. This age is much younger than most orogenic gold mineralization (ca. 60–41 Ma) in southern Tibet and indicates an orogenic gold mineralization event (ca. 20–15 Ma) related to extensional tectonics during the late-stage India-Asia continental collision.

Robust dating of Pb–Zn skarn systems by LA–ICP–MS garnet U–Pb geochronology

Lawu and Yaguila are two Pb–Zn skarn deposits in the eastern Nyainqêntanglha metallogenic belt, central Lhasa subterrane, Tibet. The genesis of Pb–Zn deposits in this belt and the crustal processes leading to their formation remain ill-constrained, mostly because of the lack of precise mineralization ages. We integrate textural information, geochemistry, and in-situ garnet U–Pb geochronology to constrain the timing and genesis of Pb–Zn mineralization. Garnets from the two deposits display oscillatory compositional zoning (And14Gr86 to And100) and contain variable U contents (0.07–5.3 ppm). Aluminum-rich garnet displays a chondrite-normalized rare earth element (REE) fractionation pattern in which LREEs are depleted relative to flattish HREE segments. In contrast, Fe-rich garnet shows LREE-enriched, relatively HREE-poor patterns with positive Eu-anomalies. Uranium concentration is correlated with total REE and Fe components in garnets, implying that U incorporation is largely controlled by coupled substitution mechanisms. The same garnets also contain measurable contents of other metals (up to hundreds of ppm), such as Sn, W, and In. The distribution and fractionation of major and trace elements in zoned garnets record periodic fluid pluses with different compositions during hydraulic fracturing. The new garnet U–Pb data show that the Yaguila deposit formed between 68.5 ± 3.4 and 65.0 ± 4.7 Ma, and the Lawu deposit formed at 54.6 ± 2.9 Ma. Within the geochronological framework of igneous rocks and Pb–Zn mineralization in the Nyainqêntanglha metallogenic belt, the new garnet ages suggest a magmatic-hydrothermal origin for related mineralization, with the causative magma likely derived from partial melting of ancient continental material due to rollback and subsequent breakoff of the Neo-Tethys oceanic slab during India–Asia continental collision. This study highlights the opportunities offered by garnet U–Pb dating for elucidating the formation age and ore genesis of base metal skarn systems.

Late Cretaceous adakitic intrusive rocks in the Laimailang area, Gangdese batholith: Implications for the Neo-Tethyan Ocean subduction

Abstract The Gangdese batholith, emplaced from the Cretaceous to the Eocene in the southern Lhasa terrane of Tibet, provides critical constraints on the tectonic-magmatic evolution of the Neo-Tethyan Ocean and the India-Asia continental collision. In this article, we report new data for the Laimailang monzogranite in the southern Gangdese, including major and trace element analyses, zircon U–Pb dating, and in situ Hf isotope analyses. In situ zircon U–Pb dating of sample yielded Late Cretaceous ages (ca. 81 Ma). The monzogranite is characterized by relatively high-silica (≥56 wt% SiO2), Na-rich, and high-Al granitoids that are characterized by high Sr, low Y and HREE contents and strongly fractionated REE patterns, with no significant Eu anomaly, indicating that they are consistent with the definition of adakite. These monzogranite have high K2O (2.92–6.5%) and negative ε Hf(t) (−2.1 to −5.4), suggesting that these rocks were likely derived from the partial melting of the lower continental crust. We conclude that the Laimailang rocks of adakitic affinity were derived due to the melting of the thickened lower continental crust in the late Cretaceous.

Cretaceous uplift history of the Tibetan Plateau: Insights from the transition of marine to terrestrial facies in central Tibet

The uplift of the Tibetan Plateau has been widely associated with India-Asia continental collision during the Cenozoic. However, recent studies have proposed that a proto-Tibetan Plateau in central Tibet was elevated during the late Mesozoic. This paper improves that understanding by focusing on the Cretaceous terrestrial strata in the Gerze area of central Tibet. Our studies suggest that the Qushenla Formation in the Nare area contains alluvial fan conglomerate intercalated with rhyolitic ignimbrite and vesicular rhyolite. Zircon UPb dating of rhyolitic ignimbrite (103.9 ± 1.4 Ma) and two granitoid boulders of conglomerate (106.0 ± 1.6 and 106.1 ± 1.5) indicate that the Qushenla Formation was deposited during the late Albian. The compositions of the Qushenla Formation conglomerate were dominated by hypabyssal intrusions on the southern Qiangtang terrane, which recorded a rapid uplift and denudation process in the late Albian, accompanied by intense volcanic eruptions. The second set of terrestrial strata is the Upper Cretaceous Jingzhushan Formation alluvial fan-braided river deposits, which can be divided into a lower conglomerate member (Member 1) and a upper sandstone member (Member 2). The large amount of bioclastic limestone fragments in the Member 1 show that they come from the Upper Cretaceous Langshan Formation in the central and northern Lhasa subterranes. While in the Member 2, abundant limestone debris, detrital zircon yielding Jurassic–Cretaceous ages with εHf(t) values between −18.6 and + 8.4, and occurrence of volcanic and ancient zircon grains indicate a volcanic-sedimentary source located in the central and northern Lhasa subterranes, with some input from the Bangong-Nujiang suture zone. This suggests that the central and northern Lhasa subterranes experienced rapid crustal uplift in the Late Cretaceous, thus ending the marine sedimentary environment since the Jurassic. Based on the temporal and spatial sedimentary characteristics of the Qushenla and Jingzhushan formations as well as the regional data, our integrated study suggests that the central plateau (Lhasa and southern Qiangtang terranes) experienced a complex uplift history during the Aptian-Turonian, with southwards stepwise growth in time and space. Furthermore, the uplift mechanism of the central plateau is complex, which was controlled by the diachronous closure of the Bangong-Nujiang Tethyan Ocean from east to west during the Early Cretaceous, and by the far-field forces associated with northwards subduction of the Neo-Tethys Ocean during the Late Cretaceous.

In situ U-Pb dating of calcite indicates a Miocene Sb-Pb mineralization event in the Sanjiang base metal metallogenic belt, SW China

The Sanjiang base metal metallogenic belt (SMB) in southwestern China is characteristic of composite mineralization during the India-Asia continental collision. However, the geodynamic settings of the composite processes of this belt are poorly understood. The Lanuoma deposit, located in the Changdu Basin in the central segment of the SMB, is a Zn-Pb-Sb polymetallic deposit. The cross-cutting relationships and paragenetic mineral associations suggest the superimposition of a late Sb-Pb mineralization event over an early Zn-dominant mineralization event. Calcite is the main gangue mineral, intergrown with late Sb-Pb minerals (e.g., boulangerite, zinkenite, sorbyite and plagionite). In situ U-Pb dating by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) was successfully carried out on calcite from this deposit, and restricts the Sb-Pb mineralization to 19.7 ± 1.6 Ma, with a mean square weighted deviation (MSWD) of 1.2. This age is approximately synchronous with that of strike-slip shearing-related leucogranites in the marginal orogens; together with previous data, this result suggests that the contemporaneous magmatism significantly contributed to the ore-forming fluids associated with the Sb-Pb mineralization. Overprinting of the Miocene Sb-Pb event in a transtensional regime on the Oligocene transpression-driven Zn mineralization resulted in the characteristic Zn-Pb-Sb assemblage at Lanuoma. Moreover, this study indicates that the SMB holds great exploration potential for Sb(-Pb) resources.

Origin of the volcanic rocks in Dianzhong Formation, central Lhasa Terrane, Tibet: implication for the genesis of syn-collisional magmatism and Neo-Tethyan slab roll-back

ABSTRACT The origin and formation of the continental collision-related magmas remain elusive. The volcanic rocks erupted during India-Asia continental collision offer an ideal opportunity to explore their genesis and geodynamic process. Here, we report new zircon U-Pb dating results and Hf isotope, whole-rock element and Sr-Nd isotope data of volcanic rocks in Linzhou Basin, central Lhasa Terrane, southern Tibet. These volcanic rocks are mainly comprised of andesites and belong to Dianzhong Formation. The timing of their formation is ca. 63–66 Ma, coeval with the India-Asia continental initial collision in the central part of southern Tibet. All these rocks show an arc-like geochemical affinity and they have more depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.705006–0.705963, εNd(t) = −1.78 to 3.52, zircon εHf(t) = 1.2–7.0) than the Cretaceous pre-collisional andesites and Eocene ancient lithospheric mantle-derived melts beneath the central Lhasa Terrane. The correlation between Mg# and CaO, TiO2, Al2O3, Sr/Y, 87Sr/86Sr, εNd(t) suggests that they were likely hybrid production between an isotopically depleted end-member and the enriched lithospheric mantle (ELM). The oceanic crust-derived melts would be the best candidate for the former end-member according to their depleted Sr-Nd isotopic compositions. The andesitic rocks of Dianzhong Formation in the central Lhasa Terrane were most likely stemmed from partial melts of altered Neo-Tethyan crust and then mixed with the ELM-derived melts. Given the Cretaceous-Tertiary upper crustal shortening, back-arc extension, and voluminous volcanism in the Lhasa Terrane, along with abrupt increasing in the magma temperature and the convergence rate between India and Asia during Palaeocene, the occurrence of the Dianzhong Formation volcanic rocks can be well explained by the Neo-Tethyan slab roll-back. The mixing between the oceanic crust- and the continental lithospheric mantle-derived melts induced by the oceanic slab roll-back would be a complimentary scenario for the formation of the syn-collisional magmatism in collisional orogeny belts.

Active Tectonics of the Nantinghe Fault in Southeastern Tibetan Plateau and its Implications for Continental Collision

Quantifying the structure and kinematics of active faults is the key to understanding the India-Eurasia continental collision. While the boundary faults of blocks in the southeastern Tibetan Plateau region have been under scrutiny for decades, relatively less attention has been paid to the active faults within the blocks. Here, we use structural, sedimentary, and geomorphic characteristics to constrain the late Quaternary kinematics and slip rates of the Nantinghe fault, a major fault within the southeastern Tibetan Plateau region. The Nantinghe fault is an active, dominantly left-lateral strike-slip fault that can be traced continuously for 500 km. We analyze the late Quaternary slip rates at five sites along the fault trace. The latest Quaternary apparent throw rates are typically less than 1 mm/a. The rate of strike-slip displacement is estimated to be an order of magnitude higher, 4.0 ± 0.6 (2σ) mm/a. Trenching suggests that active fault behavior is dominated by strike-slip faulting and reveals five earthquake events with refined information of timing in the last 5,000 years, suggesting an averaged recurrence interval of ∼1,000 a. These observations agree with GPS-derived estimates and show that late Quaternary slip rates on the Nantinghe fault are comparable to those on other major active strike-slip faults in the southeastern Tibetan Plateau. The newly-obtained slip rates on the Nantinghe fault, integrated with previous estimates on other faults in this region, provide direct evidence that favors the continuum models over the rigid block ones, on the Quaternary deformation mechanism in the India-Asia continental collision.

Crustal reworking and growth during India–Asia continental collision: Insights from early Cenozoic granitoids in the central Lhasa Terrane, Tibet

Studying granitoids can provide important knowledge on the composition of continental crust and the geodynamic processes of continental growth. Nevertheless, the mechanisms of granitoids formation during continental collision remain uncertain. In this paper, we present new zircon U–Pb–Hf isotope and whole‐rock Sr–Nd isotope compositions, and major‐ and trace‐element data for granitoids from the Chacangka area of the central Lhasa Terrane, southern Tibet. The Chacangka quartz monzonites crystallized at ca. 61 Ma, whereas the monzonites and granites were emplaced at ca. 51 Ma. The quartz monzonites are peraluminous and have high 87Sr/86Sri ratios (0.7129–0.7131), and their low εNd(t) (−7.65 to −7.10) and zircon εHf(t) (−9.3 to −7.5) values are more depleted than those of ancient crustal basement beneath the central Lhasa Terrane, which suggests that they originated from a mixed source of juvenile and ancient lower crust. Compared with the quartz monzonites, the monzonites have much higher contents of MgO (2.34–3.62 wt%) and values of Mg# (42.3–46.3) and zircon εHf(t) (−6.9 to 1.9) and show more depleted Sr–Nd isotopes, indicating that more juvenile materials were involved in their source. The granites have Sr–Nd isotope compositions that are similar to those of the coeval monzonites but have higher SiO2 contents, lower CaO and MgO contents, and lower values of Mg#, suggesting that they were differentiates of the monzonites. The elemental and isotopic compositions of the Chacangka granitoids exhibit a marked transformation from 61 to 51 Ma, probably indicating breakoff of the Neo‐Tethyan oceanic slab. The Neo‐Tethyan slab breakoff not only induced the upwelling of deep material and eruption of magma but also caused re‐melting and destruction of ancient crustal basement of the central Lhasa Terrane.

Crustal thickening versus lateral extrusion during India–Asia continental collision: 3-D thermo-mechanical modeling

Continental collision between the Indian and Eurasian plates in the past ~55 ± 5 Ma has led to the development of the Tibetan Plateau, where nearly doubled crustal thickness indicates intense lithospheric thickening during the collision. On the other hand, geological evidence indicates significant lateral extrusion of Tibetan materials around the edges of the plateau, especially around the Eastern Himalaya Syntaxis. Two end-member models, “pure shear thickening” and “tectonic escape”, emphasize respectively the vertical and lateral deformation of Tibetan lithosphere during the collision. However, their competing roles in accommodating the Tibetan continental collision remain debated. Here, we developed a series of 3-D high-resolution numerical models to systematically study strain partitioning between crustal thickening and lateral extrusion during the collision. The model focuses on eastern Tibetan Plateau, where most of the escaping tectonics occurred. The model results indicate that the amount of crustal thickening is generally several times higher than that of lateral extrusion in accommodating the lithosphere shortening. The partitioning between crustal thickening and lateral extrusion depends on the rheological contrast between the Tibetan and neighboring blocks, whereas the relic suture zones within the Tibetan lithosphere do not control the strain partitioning but affect the morphology. The southward motion of the Indochina block, resulting from extension caused by trench retreating, has major impacts on the development of large-scale lateral extrusion tectonics. The vertical variation of lithospheric rheology leads to faster motion of the crust than the mantle lithosphere in the extruding block, hence mechanical decoupling between the extruding crust and lithospheric mantle.

Metamorphic CO2 emissions from the southern Yadong-Gulu rift, Tibetan Plateau: Insights into deep carbon cycle in the India-Asia continental collision zone

Extensional rift systems provide important pathways for the release of large amounts of deeply-sourced CO2 into the atmosphere. Continental rifting zones (e.g., East African rift) are thus invoked to be important for understanding the links between CO2 outgassing and global climate change associated with continental breakup. However, deeply-sourced CO2 emissions from extensional rift systems in continental collision zones remain poorly understood. Here, we focus on hydrothermal CO2 emissions from the southern segment of the Yadong-Gulu rift (YGR), the largest extensional rift in southern Tibetan Plateau, aiming at delineating rift-related CO2 emissions from the India-Asia continental collision zone. In-situ measurements of diffuse soil CO2 emissions indicate that average soil CO2 fluxes from the Kangbu, Mengzha, and Chaduo hydrothermal fields are 40, 700, and 255 g m−2 d−1, respectively. Combined with average soil CO2 fluxes (20–437 g m−2 d−1) from central and northern YGR, we speculate a relatively steady-state CO2 degassing pattern for the entire rift. The magnitude of soil CO2 fluxes of the YGR is higher than that of representative areas of the East African rift system. Evidence from 3He/4He reveals a pure crustal origin for CO2-bearing fluids in southern YGR, while the involvement of mantle CO2 is recognized in the central and northern rift segments. We suggest that metamorphic decarbonation of crustal rocks at variable depths is the primary cause for CO2 origin in southern YGR, which differs from the magma‑carbonate interaction model of central and northern YGR. Ternary mixing calculation based on He-CO2 systematics of hydrothermal gases indicates dominant contributions from carbonate rocks to total carbon inventory, with mantle CO2 contributions absent in southern YGR but discernible in northern YGR. The characteristic high proportions of crustal CO2 (>90%) distinguish the YGR from extensional rifts fed primarily by mantle CO2 in continental rifting zones. Our results reveal a crustal-scale carbon cycle in accretionary wedge of the India-Asia continental collision zone, together with magmatic front setting of the central and northern YGR, outlining a transect of the rift-related CO2 emissions across continental collision zones. This would contribute to better understanding the role of continental assembly in deeply-sourced CO2 emissions and global carbon budget.

(U-Th)/He Thermochronology of the Indus Group, Ladakh, Northwest India: Is Neogene Cooling a Continental-Scale Thermal Event in the India-Asia Collision Zone?

Abstract The India-Asia continental collision zone archives a sedimentary record of the tectonic, geodynamic, and erosional processes that control the thermal history of the Himalayan orogenic interior since the onset of collision in early Paleogene time. In this paper, we present new (U-Th)/He thermochronometric cooling age data from 18 detrital zircons (ZHe) and 19 detrital apatites (AHe) of the early Eocene–early Miocene (ca. 50–23 Ma) continental facies of the Indus Group along the India-Asia collision zone in Ladakh, northwest (NW) India. This along-strike regional-scale low-temperature thermochronometric data set from the Indus basin is the first report of ZHe and AHe cooling ages from western and eastern Ladakh. Thermal modeling of our ZHe and AHe cooling ages indicates a postdepositional Neogene cooling signal in the Indus Group. Cooling initiated at ca. 21–19 Ma, was operational along the ~300 km strike of the collision zone in NW India by ca. 11 Ma, and continued until ca. 3 Ma. The Miocene cooling signal, also present along the India-Asia collision zone in south Tibet, is a continental-scale cooling event likely linked to increased erosional efficiency by the Indus and Yarlung Rivers across an elevated region resulting from the subduction dynamics of the underthrusting Indian plate.

Biostratigraphy and provenance analysis of the Cretaceous to Palaeogene deposits in southern Tibet: Implications for the India‐Asia collision

AbstractUpper Cretaceous to Eocene marine sedimentary sequences of the Yarlung‐Tsangpo Suture Zone (YTSZ) provide critical constraints on the initial process of the India–Asia continental collision and the closure time of the Neo‐Tethyan Ocean. New sedimentological, petrographic, biochronological and detrital zircon age data of Cretaceous–Palaeogene strata from the southern margin of the YTSZ are reported in this study. Detrital zircons from the Lower Cretaceous Gyabula Formation are dominated by Archean to Cambrian U–Pb ages, indicating a likely source originating from the Indian continent. In contrast, zircon ages from the Palaeogene Jiachala Formation are predominantly younger than 200 Ma, with a major peak of ca. 76–187 Ma, which is consistent with a source from either an intra‐oceanic island arc system or the Asian continent. Sixty species of radiolarian fossils obtained from the Gyabula, Zongzhuo and Jiachala Formations provide a regional correlation with the Cretaceous Tethyan realm and Palaeogene low‐latitude biozonation schemes based on the Unitary Association method. The Palaeogene radiolarian zonation UAZ JP10 identified in the Jiachala Formation, when combined with existing absolute age models derived from the Deep Sea Drilling Project and Ocean Drilling Program, indicates an age of ca. 61.8–61.1 Ma. Integration of the biochronological and detrital zircon age data suggests that major changes in provenance occurred at the Indian continental margin, allowing an important timing constraint on the onset of collision to be placed no later than the Danian at ca, 61.8 Ma. This study highlights the significance of focusing on distinct sedimentary sequences in collisional orogens to reconstruct the timing and processes of continental collision events.

The significance of Upper Jurassic felsic volcanic rocks within the incipient, intraoceanic Dras Arc, Ladakh, NW Himalaya

The Dras Arc is an island arc terrane located along the Indus Suture within the Ladakh Himalaya. To the north it is juxtaposed against the Eurasian Ladakh Batholith and to the south it is thrust over the Lamayuru Complex and Indian passive margin. Establishing the timing of inception and final collision of the Dras Arc is imperative to reconstructions of the Neotethyan Ocean and timing of arc-continent collisions, prior to the terminal India-Asia continental collision. We describe and date felsic tuffs and adakitic felsic volcanic rock interbedded within the dominantly basaltic-andesitic Dras volcanic complex. These felsic volcanic units yield Upper Jurassic zircon U–Pb ages of 160 ± 3 and 156 ± 1 Ma respectively, making these the oldest reported units within the Dras Arc. We also report zircon U–Pb geochronologic and whole rock geochemical results for the Kargil Intrusive Suite which intrudes the volcanic complex. Previous ages for the intrusives have been reproduced (102 ± 2 Ma and 101 ± 2 Ma), and a second, much younger phase (80 ± 1 Ma) has been identified as one of the youngest igneous phases within the Dras Arc. The presence of felsic, adakitic volcanism early in the evolution of the Dras Arc is consistent with the adolescent stages of island arc systems, in which dehydration melting of underplated arc or subducted oceanic crust generates small volumes of felsic magmas. Thus, the intraoceanic Dras Arc initiated in the Neotethyan Ocean during the Upper Jurassic, much earlier than previously reported, and possibly was active right up to collision during the late Palaeocene between 60 and 50 Ma. It is likely that the Dras Arc developed together with the Spongtang Ophiolite-Spong Arc complex and the intraoceanic Zedong terrane of Tibet, before first colliding and accreting onto the passive margin of India, prior to the terminal continental collision.

Geochemical evidence for forearc metasomatism of peridotite in the Xigaze ophiolite during subduction initiation in Neo-Tethyan Ocean, south to Tibet

Subduction initiation is one of the key processes in the operation of plate tectonics. Our understanding of this process is largely derived from geochemical studies of basaltic rocks in ophiolites at continental margins. Because of the petrological inheritance in forearc basalts, it is important to acquire geochemical signatures for fluid metasomatism of the nascent mantle wedge at forearc depths. This may be provided by the study of peridotite in ophiolites, which is least affected by tectonic emplacement of the ophiolites onto continental margins. For this purpose, we have carried out a comprehensive study of whole-rock and mineral geochemical compositions for harzburgites from the Xigaze ophiolites in the Yarlung Zangbo suture zone, southern Tibet. The results provide insights into the forearc metasomatism during subduction initiation before the closure of Neo-Tethyan Ocean for the India-Asia continental collision in the Cenozoic. The harzburgites show high spinel Cr# values and strong whole-rock depletion in CaO, Al2O3 and REE, suggesting high degrees of melt extraction (~10–20%) during seafloor spreading to form the Neo-Tethyan mid-ocean ridge basalts. They exhibit positive LILE anomalies in trace element distribution patterns, indicating metasomatism by subduction zone fluids. This is supported by the occurrence of tremolite and dolomite in some of the harzburgites. Based on the presence or absence of carbonate minerals, two types of metasomatic fluids are distinguished. One is the aqueous solutions, leading to the formation of tremolite and the enrichment of LILE in clinopyroxene. The other is the carbonate-bearing fluids, resulting in the formation of dolomites, tremolite and low Mg# (79–82) olivine. The metasomatism also results in the high contents of structural hydroxyl in pyroxenes and elevated δ18O values for the harzburgites. It is inferred that the fluid metasomatism was induced by incipient subduction of the juvenile Neo-Tethyan oceanic slab to forearc depths shortly after conversion of the Neo-Tethyan mid-ocean ridge into the trench. During this period of subduction initiation, the fluids were produced by heating dehydration of the sinking ridge crust to the forearc depths. It is these fluids that would metasomatize the overlying nascent mantle wedge, leading to superimposition of the geochemically enriched signature on the lithochemically depleted harzburgites in the ophiolites. Therefore, the forearc metasomatism of peridotite in ophiolite can be used as a geochemical proxy for subduction initiation. Furthermore, the Xigaze ophiolites were produced by the ridge-trench conversion for subduction initiation in the Mesozoic and they were emplaced onto the southern margin of the Asian continent in the Cenozoic.

Vertical thickening versus lateral extrusion during India-Asia continental collision: 3-D thermo-mechanical modeling

Vertical thickening versus lateral extrusion during India-Asia continental collision: 3-D thermo-mechanical modeling