Eastern Tethys Research Articles

Climate changes as recorded in stable carbon isotopic compositions of the Late Jurassic marine sedimentary succession in the Qiangtang Basin, Northern Tibet

The Jurassic oceanographic and climatic evolution is considered to be related to the breakup of Pangaea. As a crucial component of the carbon cycle, atmospheric CO2 concentration (pCO2) has been postulated as a main driver for climate change during the Jurassic, and concomitant changes in paleo-oceanographic conditions occurred as a result. In this study, we present a high-resolution organic matter (kerogen) carbon isotope dataset (δ13Ckerogen) from Upper Jurassic marine sedimentary rocks in the Qiangtang Basin, Tibet. The δ13Ckerogen result contains a genuine record concerning the response of the eastern Tethys to exogenic carbon cycle perturbations in both marine and atmospheric reservoirs and is also consistent with the high-resolution bulk carbonate and organic matter carbon-isotope records from the Atlantic and western Tethys. The relative fractionation of carbon isotopes in organic matter vs. carbonate species, defined as Δ13C (δ13Ccarb-δ13Ckerogen), and the secular trend of atmospheric pCO2 over the Late Jurassic that is calculated from the high-resolution δ13Ckerogen values indicate a cold Callovian-Oxfordian transition, a long-term increasing but fluctuating Kimmeridgian and a prominent early Tithonian cooling event (ETCE). The pronounced temperature plateau during the late Kimmeridgian-early Tithonian was contemporaneous with the occurrence of major magmatic events during the Late Jurassic, while the ETCE has been possibly attributed to major changes in oceanic circulation patterns. Additionally, reconstructed atmospheric pCO2 values show very small differences to values using phytane- and stomatal density-based pCO2 calculations, providing an alternative estimate for accurate identification of the paleoclimatic framework of this enigmatic interval in the Mesozoic.

The Anisian (Middle Triassic) brachiopods from the southern Qilian Mountains, north‐western China

AbstractThe Anisian (Middle Triassic) witnessed the diversification of brachiopods after the Permian–Triassic extinction. The Anisian brachiopod fauna from the southern Qilian Mountains in north‐western China has the highest taxonomic diversity at species level among coeval faunas worldwide. Nevertheless, many taxa from this remote region remain poorly defined, and therefore require emendation. Here we describe 23 species (including two uncertain species) in 15 brachiopod genera based on well‐preserved carbonate and silicified specimens collected from the Anisian Dajialian and Qieermagou formations in Tianjun, southern Qilian Mountains. Two new genera (Crenulatomargus, Tianjunospina) and six new species (Costirhynchopsis xui, Crenulatomargus terebratuliformis, Schwagerispira elegans, Lepismatina? inusitata, Tianjunospina junheensis, Qilianoconcha circularis) are erected. The diagnoses of Triasorhynchia, Yangkongia, Sinucostella, Aequspiriferina, Qilianoconcha and Parantiptychia are emended. Also, the genus Eoantiptychia and the family Triasorhynchiidae are regarded as junior synonyms of Qilianoconcha and Tetrarhynchiidae, respectively. After systematic description and taxonomic emendation, 56 species (including seven uncertain species) in 26 genera are confirmed in the southern Qilian brachiopod fauna. Network analysis of 14 brachiopod faunas worldwide shows that six groups were detected during the Anisian (Qingyan, southern Qilian – western Qinling, Himalaya – northern Siberia, central Qinghai, New Zealand, and Europe/western Tethys). Compared with those from the western Tethys, the eastern Tethyan faunas are less allied to each other. Except for the assemblage from the adjacent western Qinling area, other coeval faunas have much lower affinities with the Qilian fauna.

Increasing primary productivity in the oligotrophic Tethyan coastal ocean during the Paleocene-Eocene warming episode

The coastal upwelling zones, occupying only ~0.5% of the global ocean, account for ~10% of the global primary productivity. The CO 2 fixation by primary producers amplifies in the upwelling zones during global warming due to the higher nutrient supply. Based on the presumption that the nutrient-deficient coastal ocean is less productive, the state of the oligotrophic coastal ocean is often neglected in the productivity-climate change studies. The present study investigated the changes in the primary productivity, redox condition, and nutrient content, using algal abundance, total organic carbon, and various major, trace, and rare earth elements and yttrium (REY) proxies, of the oligotrophic equatorial eastern Tethyan coastal ocean across the Paleocene-Eocene Thermal Maximum (PETM), a prominent paleo-global warming event. Despite the lower nutrient (lower Ni EF , Cu EF , and Zn EF ) contents, and invariable salinity, p H, and light conditions, the PETM interval shows extensive growth of coralline red algae in the hypoxic-oxic water column. Based on these observations, and inferences drawn from the previous laboratory experiments, conducted on the algal growth in varying p CO 2 by others, we postulate that the increased atmospheric CO 2 concentrations during the PETM probably enhanced the primary productivity of the oligotrophic Tethyan coastal ocean. If so, then the oligotrophic coastal ocean may be considered as an effective CO 2 sink and likely to play a pivotal role in carbon cycle-climate connection studies. • Response of the oligotrophic coastal ocean during global warming is grossly unknown. • Tethyan low-latitude, shallow-marine sections record PETM in carbonate sequence. • Primary productivity (coralline algae) increased during the PETM despite low nutrient. • The higher atmospheric p CO 2 might have triggered the excessive growth of algae. • Oligotrophic coastal ocean can act as an effective carbon sink.

Digital paleogeographic reconstruction of the eastern Tethyan tectonic domain from the Middle Permian to the Middle Triassic

Paleogeographic reconstruction studies the evolution of the earth's surface tectonic process, the pattern of land and sea, climate, and biology in time and space in geological history. With the advent of the big data era, the accuracy of the digital paleogeographic reconstruction is essential. Although there are various global paleogeographic reconstruction models, there is still a lack of paleogeographic reconstruction integrating tectonic, lithofacies, paleontology, and paleoclimate data. In this paper, a case study of the comprehensive paleogeographic reconstruction is carried out in the Middle Permian-Middle Triassic eastern Tethyan tectonic domain, which is highly concerned and rich in geological data accumulation. The digital paleogeographic map is completed by the combination of automatic mapping with machine learning and manual correction. We use the newly upgraded paleomagnetic, geological and paleontological data to reconstruct the paleoposition of the East Asian blocks at 260, 250 and 240 Ma, which shows the Paleo-Tethys Ocean (PTO) had a wider east-west range than the previous version due to adopting the method of Torsvik et al. (2008) to reconstruct the paleolongitude of South China at 260 Ma. The new model shows the PTO was semi-closed with several narrow seaways and was divided into north and south branches at 260 Ma. Based on the updated lithofacies paleogeographic map data of China and marine fossils data, we updated the paleogeographic map of the Eastern Tethyan tectonic domain on the basis of the global paleogeographic map, combining automatic mapping with machine learning and manual revision of paleotopography and paleocoastline. Based on the new tectonic paleogeographic reconstruction, we revised the paleoclimate zonation pattern of the East Tethys ocean using climate-sensitive lithologies.

Evolution of Ocean Circulation in the North Atlantic Ocean During the Miocene: Impact of the Greenland Ice Sheet and the Eastern Tethys Seaway

AbstractThe Modern Ocean is characterized by the formation of deep‐water in the North Atlantic Ocean (i.e., NADW). This feature has been attributed to the modern geography, in which the Atlantic Ocean is a large basin extending from northern polar latitudes to the Southern Ocean, the latter enabling the connection of the otherwise isolated Atlantic with the Pacific and Indian Oceans. Sedimentary data date the establishment of the NADW between the beginning of the Eocene (∼49 Ma) and the beginning of the Miocene (∼23 Ma). The objective of this study is to quantify the impact of Miocene geography (∼20 Ma) on NADW using new simulations performed with the Earth System Model IPSL‐CM5A2. We specifically focus on the closure of the Eastern Tethys Seaway (ETS), dated between 22 and 14 Ma, which allowed the connection between the Atlantic and Indian Oceans, and on the Greenland Ice Sheet, whose earliest onset remains open to discussion but for which evidence suggest a possible existence as early as the Eocene. Our results show that the closure of the ETS does not appear to impact the establishment of NADW, because waters from the Indian Ocean do not reach the NADW formation zone when the seaway is open. Conversely, the existence of an ice sheet over Greenland strengthens the formation of NADW owing to topography induced changes in wind patterns over the North Atlantic, which in turn, results in a salinification of the North Atlantic and Nordic Seas, and in an increase in deep‐water formation.

Paleoenvironmental and Bio-Sequence Stratigraphic Analysis of the Cretaceous Pelagic Carbonates of Eastern Tethys, Sulaiman Range, Pakistan

The Cretaceous pelagic carbonate succession, i.e., Goru Formation was studied in the Chutair Section, Sulaiman Range, representing part of the eastern Tethys for the paleoenvironment and bio-sequence stratigraphy. Eight planktonic foraminiferal biozones are identified which include: 1. Muricohedbergella planispira Interval Zone; 2. Ticinella primula Interval Zone; 3. Biticinella breggiensis Interval Zone; 4. Rotalipora appenninica Interval Zone; 5. Rotalipora cushmani Total Range Zone; 6. Whiteinella archeocretacea Partial Range Zone; 7. Helvetoglobotruncana helvetica Total Range Zone; and 8. Marginotruncana sigali Partial Range Zone representing Albian-Turonian age. The petrographic studies revealed five microfacies: 1. Radiolarians-rich wacke-packestone microfacies; 2. Radiolarians-rich wackestone microfacies; 3. Planktonic foraminiferal wacke-packestone microfacies; 4. Planktonic foraminiferal wackestone microfacies; and 5. Planktonic foraminiferal packestone microfacies; indicating deposition of the Goru Formation in outer-ramp to deep basinal settings. Based on the facies variations and planktonic foraminiferal biozones, the 2nd and 3rd order cycles are identified, which further include six transgressive and five regressive system tracts. The sea level curve of the Goru Formation showed fluctuation between outer-ramp and deep-basin, showing the overall transgression in the 2nd order cycle in the study area, which coincides with Global Sea Level Curve; however, the 3rd order cycle represents the local tectonic control during deposition of the strata.

Shallow Water Records of the PETM: Novel Insights From NE India (Eastern Tethys)

AbstractThe Paleocene‐Eocene Thermal Maximum (PETM) is associated with major extinctions in the deep ocean, and significant paleogeographic and ecological changes in surface ocean and terrestrial environments. However, the impact of the associated environmental change on shelf biota is less well understood. Here, we present a new PETM record of a low paleolatitude shallow‐marine carbonate platform from Meghalaya, NE India (eastern Tethys). The biotic assemblage was distinctly different to other Tethyan PETM records dominated by larger benthic foraminifera and calcareous algae both in the Paleocene and Eocene. A change in taxa and forms indicating deeper waters with a concurrent decrease in abundance of shallow water algae suggests a sea‐level rise during the onset of the PETM. The record is lacking the ecological change from corals to larger foraminiferal assemblages and the Lockhartia dominance, characteristic of several other sections in the Tethys. Comparison with a global circulation model (GCM) indicates high regional temperatures in the Thanetian which may have excluded corals from the region. Furthermore, the regional circulation pattern is isolating the site from the wider Paratethys. Our study highlights the need for a diverse global perspective on shallow‐marine response to the PETM and the strength of coupling data to global climate models for interpretation.

A new lycophyte megaspore, Paxillitriletes permicus, from the upper Permian of Southwest China

Lycophytes were an important group of plants in the late Permian (Lopingian) vegetation of Southwest China. However, our understanding of these lycophytes is based mostly on the study of megafossil specimens. Here, we describe a unique lycophyte megaspore, Paxillitriletes permicus Sui, McLoughlin et Feng sp. nov., from the Lopingian Xuanwei Formation of Yunnan Province, Southwest China. This trilete megaspore is characterized by prominent membraneous triradiate flanges, long bifurcate spines, an arcuate ridge expanded into a zona, and triangular to polygonal reticulate sculpture. Ultrastructural analysis reveals that the megaspore wall consists of four layers. The innermost layer is the foot layer, which forms a thin and solid basal lamina. It is covered by a dense layer containing small and parallel sporopollenin grains. A thick spongy layer is developed exterior to the dense layer and consists of elongate, curved, and intersected sporopollenin units with porous zones. The outermost layer is dense, of variable thickness, and forms the processes. Morphological and ultrastructural features indicate that the new megaspore belongs to a herbaceous isoetalean. This is the first detailed investigation of megaspores from the Lopingian of China using scanning and transmission electron microscopy. Our discovery represents the oldest occurrence of Paxillitriletes, and adds to the diversity of late Permian lycophytes in the Cathaysian Flora in the paleotropics of the eastern Tethys Ocean.

Evaluation of chemical weathering proxies by comparing drilled cores versus outcrops and weathering history during the Permian–Triassic transition

Continental records of the Permian–Triassic boundary are characterized by intense chemical weathering, likely due to global warming and acid rainfall associated with volcanism. However, the weathering history throughout the late Permian has not been interrogated in time-equivalent continental successions because of the lack of stratigraphically continuous deposits. Here, we present comparative investigations of outcrops and drilled cores from upper Permian to lowest Triassic terrestrial deposits from the Taoshujing Section, South China. These sediments have a mafic-dominated source with insignificant sorting and recycling during deposition, as documented by the invariable TiO2/Al2O3, Zr/Ti, Th/Sc, Zr/Sc, and Cr/Th ratios, and low values of Th/Sc and Cr/Th ratios. However, different diagenetic effects on outcrops and drilled cores have generated counterintuitively low values of commonly used chemical weathering proxies. Potassium enrichment, recognized from the A-CN-K diagram produced for portions of the outcrop, is also verified by the strongly negative correlations between CIA, CIX, and the molar contents of K2O (r = −0.90 and −0.92, respectively), as well as clay mineralogy analysis. However, the reduced CIA, CIW, and PIA values for the drilled core samples primarily result from an insufficient subtracting of CaO in the secondary carbonates. Using two modified proxies (CIX and CPA), the decline observed in the lower Xuanwei Fm. was probably caused by the cooling climate during the early Wuchiapingian. Nevertheless, the lower values from the Kayitou Fm. to the basal Dongchuan/Feixianguan Fm. likely indicated weak chemical weathering intensity due to physical denudation and subsequent rapid deposition under rapid warming and short fluid-rock reaction time conditions. This weak chemical weathering might have been intensified by drier conditions and increased seasonality in the sediment source area and tectonics related to the closure of the eastern Tethys Ocean. Our work is the first comparative study on chemical weathering proxies for drilled core and outcrop samples from the Late Permian to Early Triassic, a time period which included the most severe bio-crisis and global environmental change in the Phanerozoic. These results highlight that non-weathering factor alterations on commonly used chemical weathering proxies are essential to evaluate with multiple techniques. Compared to the drilled cores, samples collected from the subsurface by excavation could not avoid later weathering alteration and should be used with caution to uncover the past chemical weathering history of an area.

Early Jurassic Mafic Magmatism in the Eastern Tethyan Himalaya, Southern Tibet

Early Jurassic Mafic Magmatism in the Eastern Tethyan Himalaya, Southern Tibet

VALANGINIAN RADIOLARIANS OF NE SARDINIA (ITALY) IN THE FRAME OF THE WEISSERT EVENT

This article presents the very rich radiolarian fauna from the S’Ozzastru section of the Siniscola Marl crop-ping out in the northern part of Mt Albo massif (NE Sardinia, Italy). The formation was ascribed to the lower part of the upper Valanginian on the basis of the ammonite fauna indicating the Verrucosum Zone and the nannofossil con-tent suggesting the Zone NK3. Due to the scarcity of pantanelliids and especially of the absence of Cecrops septem-poratus, radiolarian marker of the upper Valanginian, radiolarians can only indicate a Valanginian age. Investigations for integrated litho-, bio-, and chemostratigraphy allowed the identification of the Valanginian Weissert Oceanic An-oxic Event (OAE), testified by a positive carbon isotope excursion (CIE) with values up to 2.98 ‰. Radiolarian study includes not only a determination of the species for biostratigraphic interpretations, but also a revision of the system-atics of the radiolarians occurring in the section. One new family (Crococapsidae), 3 new genera (Apiaromitra, Mi-crosandwichia and Ocellicapsa) and 43 new species are erected. The fauna has some elements in common with the Valanginian faunas from the Mediterranean area and many more with those from Japan, proving a wide circulation between the western and eastern Tethys.

Palynological record of the Carnian Pluvial Episode from the northwestern Sichuan Basin, SW China

The prevailing arid Late Triassic climate was interrupted by a humid Carnian Pluvial Episode (CPE) during the mid-Carnian period. In this study, a palynological study was conducted for the Ma'antang Formation (Carnian) from the northwestern Sichuan Basin, SW China, in the eastern Tethys. The study reveals dominance of terrestrial palynomorphs, and two palynological assemblage zones were identified. Palynofloras are well-presented by dominant ferns (especially Dipteridaceae/Matoniaceae), with less abundant lycopsids, conifers, cycadophytes/ginkgophytes and seed ferns. Overwhelming predominance of wet Lowland SEG and hygrophyte elements, and increased Lowland/Hinterland as well as hygrophyte/xerophyte ratios observed in Units 2–4 of the Ma'antang Formation, suggesting intensified humidity during the Julian 2 in this area. This study presents the first palynological evidence for vegetation changes in South China during the CPE, and correlates well with observations from North China as well as western Tethys, thus further supports worldwide impact of humid climate associated with the CPE.

Carbon-isotope excursions in the Norian stage (Upper Triassic) of the Baoshan terrane, western Yunnan, China

The biostratigraphy and carbon stable isotopes of the Norian Stage (Upper Triassic) are well studied in the western Tethys, but little information is available from the eastern Tethys. Therefore, we studied the Hongyan-B (HYB) section in the Baoshan terrane, western Yunnan Province, SW China, which was located in the eastern Tethys during the Late Triassic. The HYB section was investigated for conodonts, radiolarians, total organic carbon, carbonate carbon and oxygen stable isotopes (δ13Ccarb and δ18Ocarb), and carbon isotopes of organic matter (δ13Corg). The Mockina slovakensis and Mockina bidentata conodont biozones proposed in the HYB section are associated with two radiolarian zones of the Sevatian substage, the Praemesosaturnalis multidentatus and Praemesosaturnalis pseudokahleri zones (TR8A and TR8B Sugiyama zones). This biostratigraphy places the Alaunian-Sevatian substage boundary (middle-upper Norian) at meter 23 of the HYB section. Unfortunately, the δ13Ccarb and δ18Ocarb values suggest that they might have been influenced by diagenesis. However, the δ13Corg record preserves multiple carbon-isotope excursions across the Alaunian-Sevatian boundary that can be correlated with other coeval global sections. The carbon-isotope excursions registered at this middle-upper Norian boundary are perhaps related to the major outgassing of light carbon during the emplacement of Angayucham flood basalts.

Biostratigraphic correlation of the transition of Maastrichtian–Thanetian based on calcareous nannofossils and planktonic foraminifera in Fars province, Zagros (Eastern Tethyan realm)

Biostratigraphic correlation of the transition of Maastrichtian–Thanetian based on calcareous nannofossils and planktonic foraminifera in Fars province, Zagros (Eastern Tethyan realm)

Taxonomy and paleobiogeographic implication of Glabrobournonia Morris and Skelton (Hippuritida, Radiolitidae) from the Late Cretaceous Yigeziya Formation, southwestern Tarim Basin

A re-examination of the specimens that were identified as Biradiolites minor Pojarkova from the late Campanian to early Maastrichtian middle member of the Yigeziya Formation of southwestern Tarim Basin reveals that they should be assigned to the genus Glabrobournonia Morris and Skelton. Glabrobournonia is a genus of radiolitids characterized by indented radial bands, salient ridges on the shell margins and absence of fine ribs on the surface of the right valve. Apart from southwestern Tarim Basin, Glabrobournonia minor (Pojarkova) has also been recorded from the late Campanian of Fergana and Alai basins. The central Asian, late Campanian to early Maastrichtian G. minor differs from the late Campanian to Maastrichtian, eastern Arabian type species Glabrobournonia arabica Morris and Skelton in the flat left valve and an additional fourth ridge on the junction of the dorsal and posterior sides of the right valve. Biradiolites ingens (Des Moulins) could be the direct ancestor of Glabrobournonia. The paleogeographic distribution of Glabrobournonia suggests that this genus dispersed to central Asia from the late Campanian time, becoming widely distributed in the eastern Tethyan region rather than endemic to eastern Arabia. Correspondingly, specimens belonging to Gyropleura yielded from the same bed as G. minor in southwestern Tarim Basin, are similar to the specimens which were attributed to the eastern Arabian Gyropleura sp.; Campanian to early Maastrichtian Osculigera specimens described from the Yigeziya Formation are comparable with those known from the Campanian–Maastrichtian of Iran, Afghanistan and eastern Arabia. The similarity of the rudist assemblages between central Asia and eastern Arabia suggests a faunal connection and affinity between the north and south margins of the eastern Tethyan realm during Campanian to early Maastrichtian times.

A giant barnacle (Crustacea, Cirripedia, Thoracica) from the Upper Cretaceous (Maastrichtian) of Canada revisited

The cirripede Calantica? saskatchewanensis Russell, 1967, from the Maastrichtian Bearpaw Formation of Saskatchewan (Canada), is redescribed on the basis of new material and transferred to the genus Pachyscalpellum Buckeridge, 1991. This considerably extends the known range of the genus from the eastern Tethys (Australasia, Tunisia) to the northern part of the Western Interior Seaway of North America. Species of the genus achieved an exceptionally large size for stalked barnacles, with a projected possible height (capitulum and peduncle) of 150–200 mm.

The Proto- and Palaeo-Tethys tectonic evolution in Southeastern Tibetan Plateau: Constraints from detrital zircon dating of metasedimentary rocks from the Diancang Shan complex

ABSTRACT Metamorphic rocks from the Diancang Shan (DCS) complex along the Ailao Shan-Red River belt between the South China plate and Simao-Indochina block are studied in this contribution to constrain the Proto- to Palaeo-Tethys tectonic evolution of the eastern Tethys domain. LA-ICP-MS zircon U-Pb geochronological dating is conducted in this study on the metasedimentary rocks (garnet-biotite-plagioclase gneisses, biotite-quartz schists and phyllonites) from different stratigraphical horizons (Proterozoic and Triassic) of the DCS complex. The analyses yielded three major age peaks at ca. 248 Ma, ca. 441 Ma, ca. 1000–900 Ma and one subordinate age peak at ca. 828 Ma. The youngest ages for the samples help to constrain the maximum ages of deposition of the complex at Early Carboniferous and Late Triassic, respectively. The existence of the diagnostic age peak at ca. 440 Ma and the euhedral-subhedral morphology of the detrital zircons in the Palaeozoic and Mesozoic strata may indicate that there were early Palaeozoic magmatic activities along the eastern margin of the Simao-Indochina block. At the meantime, the age peak at ca. 440 Ma was never reported in the early Palaeozoic strata from the western margin of the South China plate, which supports the existence of an early Palaeozoic rift zone, i.e., the Proto-Tethys Jinsha Jiang-Ailao Shan-Song Ma rift zone, being consistent with geochemical discriminations. Furthermore, we conclude, from the significant difference between detrital zircon age data of Silurian to Devonian sedimentary rocks from the Simao-Indochina block and South China plate, the opening of the Palaeo-Tethys ocean since Early Carboniferous, the absence of evidence for the Palaeozoic orogeny, and the successive sedimentation from the Cambrian to Triassic along the western margin of the South China plate, that the Proto-Tethys rifting and subsequent Palaeo-Tethys ocean evolution along the Jinsha Jiang-Ailao Shan-Song Ma zone formed actually a complete ‘Wilson Cycle’ from early Palaeozoic to the Late Triassic.

Comparison of Santonian/Danian strata based on calcareous nannofossils and planktonic foraminifera at the Zagros Basin, SW Iran, Eastern Tethyan realm

Comparison of Santonian/Danian strata based on calcareous nannofossils and planktonic foraminifera at the Zagros Basin, SW Iran, Eastern Tethyan realm

Granitic Magmatism in Eastern Tethys Domain (Western China) and their Geodynamic Implications

AbstractWestern China locates in the eastern section of the Tethys domain, granitic rocks in this region with variable formation ages and geochemistry record key information about the crust‐mantle structure and thermal evolution during the convergent process of Tethys. In this study, we focus on some crucial granitic magmatism in the western Yangtze, Qinling orogen, and western Sanjiang tectonic belt, where magma sequence in the convergent orogenic belt can provide important information about the crust‐mantle structure, thermal condition and melting regime that related to the evolution processes from Pre‐ to Neo‐Tethys. At first, we show some features of Pre‐Tethyan magmatism, such as Neoproterozoic magmatism (ca. 870–740 Ma) in the western margin of the Yangtze Block were induced by the assembly and breakup of the Rodinia supercontinent. The complication of voluminous Neoproterozoic igneous rocks indicated that the western Yangtze Block underwent the thermodynamic evolution from hot mantle‐cold crust stage (ca. 870–850 Ma) to hot mantle and crust stage (ca. 850–740 Ma). The Neoproterozoic mantle sources beneath the western Yangtze Block were progressively metasomatized by subduction‐related compositions from slab fluids (initial at ca. 870 Ma), sediment melts (initial at ca. 850 Ma), to oceanic slab melts (initial at ca. 825–820 Ma) during the persistent subduction process. Secondly, the early Paleozoic magmatism can be well related to three distinctive stages (variable interaction of mantle‐crust to crustal melting to variable sources) from an Andeans‐type continental margin to collision to extension in response to the evolution of Proto‐Tethys and final assembly of Gondwana continent. Thirdly, the Paleo‐Tethys magmatism, Triassic granites in the Qinling orogenic display identical formation ages and Lu‐Hf isotopic compositions with the related mafic enclaves, indicate a coeval melting event of lower continental crust and mantle lithosphere in the Triassic convergent process and a continued hot mantle and crust thermal condition through the interaction of subducted continental crust and upwelling asthenosphere. Finally, the Meso‐ and Neo‐Tethyan magmatism: Early Cretaceous magmatism in the Tengchong Block are well responding to the subduction and closure of Bangong‐Nujiang Meso‐Tethys, recycled sediments metasomatized mantle by subduction since 130 Ma and subsequently upwelling asthenosphere since ca. 122 Ma that causes melting of heterogeneous continental crust until the final convergence, this process well recorded the changing thermal condition from hot mantle‐cold crust to hot mantle and crust; The Late Cretaceous to Early Cenozoic magmatism well recorded the processes from Neo‐Tethyan ocean slab flat subduction, steep subduction, to initial collision of India‐Asia, it resulted in a series of continental arc magmatism with enriched mantle to crustal materials at Late Cretaceous, increasing depleted and/or juvenile materials at the beginning of early Cenozoic, and increasing evolved crustal materials in the final stage, implying a continued hot mantle and crust condition during that time. Then we can better understand the magmatic processes and variable melting from the mantle to crust during the evolution of Tethys, from Pre‐, Paleo‐, Meso‐, to Neo‐, both they show notably intensive interaction of crust‐mantle and extensive melting of the heterogeneous continent during the final closure of Tethys and convergence of blocks, and thermal perturbation by a dynamic process in the depth could be the first mechanism to control the thermal condition of mantle and crust and associated composition of magmatism.

Depositional and diagenetic constraints on the quality of shale‐gas reservoirs: A case study from the Late Palaeocene of the Potwar Basin (Pakistan, Eastern Tethys)

Unconventional shale‐gas reservoir assessment concurs numerous exploration challenges, often related to their complex lithology, which reflects secular changes in depositional environment(s) as well as spatially variable diagenetic overprinting. This study combines a range of methods (sedimentological, petrographical, CL‐microscopy, X‐ray diffraction, TOC (total organic carbon), TON (total organic nitrogen) and δ13Corg and δ15Norg stable isotopes) to address the controls of lithofacies, palaeo‐depositional environment(s) and diagenesis on the shale‐gas reservoir potential of the Late Palaeocene Patala Formation in the Potwar Basin of Pakistan. This formation records sediment accumulation in a shallow, mixed siliciclastic‐carbonate shelf environment. Sedimentological and X‐ray diffraction (XRD) analyses show that the formation is primarily composed of an alternation of carbonaceous, siliceous, calcareous and argillaceous mudstone lithofacies. Detrital assemblages, including grains of quartz, apatite, calcite, chlorite, as well as organic matter (OM) and clay minerals with auxiliary (authigenic) pyrite, dominate the formation. High terrigenous influx is represented by the abundance of siliciclastics and terrigenous OM. The δ13Corg and δ15Norg proxies reflect dysoxic to anoxic palaeo‐environmental conditions, which promoted preservation of mixed marine and terrigenous OM, a setting considered to be indicative for high shale‐gas potential. The organic‐rich siliceous and carbonaceous mudstone lithofacies are considered to be the most prospective intervals, while calcareous and argillaceous mudstones are considered least promising in terms of reservoir quality. Spatially variable eo‐, meso‐ and telo‐diagenetic features such as compaction, cementation, stylolitization, dissolution and re‐precipitation are superimposed upon the depositional fabric, which locally affected the reservoir quality of the Patala Formation. The results of this study strongly suggest that the organic‐ and quartz‐rich mudstone lithofacies constitute “sweet spots” whose occurrence in space and time may be traced to design a strategy for shale‐gas exploration in the Potwar Basin of Pakistan and in analogous settings elsewhere.