A dominantly NW-SE directed extensional tectonics in the Early Cretaceous significantly reworked the Late Permian-Triassic orogenic framework of the Dabie orogenic belt. The North Dabie complex (NDC) is the principal domain recording this tectonic event. However, the precise structure-kinematic architectures, particularly those observed in the ductile regime, along with the respective time scales for different extensional stages, have not been adequately established. This significantly impedes our comprehensive understanding of the extensional style and deformation history in the North Dabie complex. To better address these issues, we conducted a systematic structural study and LA-ICP-MS zircon U-Pb dating of the pre-, syn-, and post-kinematic intrusions and syn-kinematically metamorphosed high-grade gneisses/migmatites of the NDC. Our results demonstrate that the extensional deformation in the NDC may initiate at ca. 144 Ma, which is characterized by a pervasive NW-SE oriented coaxial plastic flow in the ductile regime of the middle-lower crust. A large-scale detachment processing zone subsequently started activating at ca. 140 Ma at the upper-middle level of the middle crust, and concentratedly accommodated the extensional strain by top-to-NW ductile shearing. Locally, there was uprising of sub-magmatic flow in the atatexite-diatexite from the deeper lower crust taking place in the manner of top-to-outward shearing as early as ca. 137 Ma. This composite process of extension manifests vertical strain partitioning across the ductile middle-lower crusts and progressive strain localization during the lithospheric thinning. The NW-SE orientation dominated extensional tectonics was strongly driven by the westward subduction of the Paleo-Pacific oceanic plate during the Late Mesozoic.
Abstract Palaeogene environmental evolution in East Asia remains ambiguous. Here we present integrative work including magnetostratigraphy, grain‐size, geochemistry, and clay mineralalogy from a 1609 m‐thick fluviolacustrine sequence in eastern China. The results reveal two periods of tectonic control alternating with three periods of climatic control on the sedimentary evolution. Tectonic activity in the study area, as revealed by particle coarsening and reduced weathering, occurred during 65.6–59 Ma and strengthened in Asia during 55–54 Ma in response to the India‐Eurasia collision. Weathering gradually enhanced in East Asia during 59–55 Ma, probably caused by global warming. Continuous global warming during 54–50.5 Ma is responsible for enhanced aridification in East Asia. From 50.5 to 37.6 Ma, global cooling weakened evapotranspiration and increased westerlies‐derived moisture. Both aspects increased effective moisture and chemical weathering in East Asia. These results shed light on how alternating tectonism and climate change impacted environmental evolution in Asia during the Palaeogene.
The accretionary complex (AC) in the North Qilian belt comprises coherent and chaotic units consisting of bedded cherts, pelagic mudstone, shale, turbidites, basalt, limestone, blueschist, eclogite lenses and ophiolitic mélange. Cherts from the Donggoukou and Biandukou outcrops in the north of the blueschist belt contain abundant Middle Ordovician radiolarians together with rare conodonts. Well-preserved radiolarians also occur in cherts associated with high-pressure/low-temperature rocks in the Baijingsi AC outcrop. Conodonts of Floian–Dapingian age and Middle Ordovician radiolarians also occur in the Shihuigou AC. Geochemical analysis of 23 cherts reveals variable SiO 2 contents (74.56–97.16 wt%) and high mean Al/(Al + Fe + Mn) ratios ranging from 0.35 to 0.85, indicating a non-hydrothermal origin. Ce/Ce* and La N /Yb N ratios of 0.70–1.22 and 0.67–1.59 respectively are high and variable, similar to those of associated muddy siltstone (0.59–0.96 and 1.14–1.55, respectively), suggesting near-trench deposition with associated terrigenous input. Together with the metamorphic ages of blueschists and eclogites, the North Qilian belt AC formed by accretion of ocean plate stratigraphic successions in response to subduction of the Proto-Tethyan Ocean prior to 450 Ma. Supplementary material: Appendices 1-6 are available at: https://doi.org/10.6084/m9.figshare.c.5418275 Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts
Abstract Many ophiolite complexes like those of Oman and New Caledonia represent fragments of ancient oceanic crust and upper mantle generated at supra‐subduction zone environments and have been obducted onto the adjacent rifted continental margin together with the accretionary complexes and intra‐oceanic arcs. The Lajishan ophiolite complexes in the Qilian orogenic belt along the NE edge of the Tibet‐Qinghai Plateau are one of several ophiolites situated to the south of the Central Qilian block. Our geological mapping and petrological investigations suggest that the Lajishankou ophiolite complex consists of serpentinite, wehrlite, pyroxenite, gabbro, dolerite, and pillow and massive basalts that occur in a series of elongate fault‐bounded slices. An accretionary complex composed mainly of basalt, radiolarian chert, sandstone, mudstone, and mélange lies structurally beneath the ophiolite complex. The Lajishankou ophiolite complex and accretionary complex were emplaced onto the Qingshipo Formation of the Central Qilian block which shows features typical of turbidites deposited in a deep‐water environment of passive continental margin. Our geochemical and geochronological studies indicate that the mafic rocks in the Lajishankou ophiolite complex can be categorized into three distinct groups: massive island arc tholeiites, 509 Ma back‐arc dolerite dykes, and 491 Ma pillow basaltic and dolerite slices that are of seamount origin in a back‐arc basin. The ophiolite and accretionary complex constitute a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt during the early Paleozoic southward subduction of the South Qilian Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block.
The metamorphism of the Central Qilian Block in the northeastern Tibetan Plateau records a complete tectonic history of the Qilian Orogen. Here, results of structural measurements, geochronological data, and thermobarometry for metamorphic rocks in the Central Qilian Block were presented, which trace the tectonic evolution of the Qilian Orogen. New U–Pb dating of detrital zircon from one paragneiss shows a main age population between c . 1500 and c . 1250 Ma, with the youngest age of 1085 Ma. This unit was intruded by an orthogneiss which has a U–Pb weighted mean age of 920 ± 18 Ma. Together with c . 1200–1000 Ma ages from inherited zircon cores in amphibolites, these results indicate that the protoliths of the Huangyuan Group were formed during the Mesoproterozoic and Neoproterozoic. Rims of these zircons obtain tightly constrained and concordant ages ranging from c . 459 to c . 427 Ma, with a weighted mean age of c . 450 Ma. Phase equilibrium modelling and conventional thermobarometry jointly indicate high‐temperature/medium‐pressure HT–MP metamorphism along a clockwise pressure–temperature ( P–T ) path at c . 450 Ma, passing through prograde conditions of 7.8–8.0 kbar and 620–650°C to peak conditions of ~7 kbar and ~780–800°C. Together with documented widespread metamorphism, magmatism, and ductile shear belts, these new results reassert that the Central Qilian Block experienced a three‐stage tectono‐metamorphic evolution during the Early Palaeozoic and the HT–MP metamorphism of Huangyuan Group was developed by a continental collision geodynamic setting with coeval mafic and granitic magmatism.
The Changning‐Menglian Belt in western Yunnan, China, is one of the key regions to decode the evolution of Palaeo‐Tethys. However, the work has been hampered considerably by the loose definition and composite nature of some stratigraphic units in the belt. These units normally have varied lithological components (slices of sequences), most of which yield scarce fossils. An example is the ‘Middle‐Upper Devonian (D 2‐3 )’ unit, which was established to accommodate a series of cherts, shales, siltstones and sandstones, occurring rather frequently in the central zone of the Belt. Its age assignment was mainly based on the Late Devonian conodonts from some siliceous shale intervals. However, the ‘D 2‐3 ’ unit contains strata of ages other than Middle‐Late Devonian, as has been indicated by a few studies in the Chahe‐Manxin area. The Late Permian radiolarians have been obtained from bedded cherts of the ‘D 2‐3 ’ in Paqiu area which provided the first solid evidence for the presence of Late Permian beds in the ‘D 2‐3 ’ in this area. Radiolarians from two chert outcrops about 3 km apart can be assigned to two Wuchiapingian‐Changhsingian assemblages, namely, the Albaillella levis and Albaillella protolevis assemblages, respectively, with the former being slightly younger than the latter. Statistical analysis of Permian radiolarians from different localities in the Belt shows that Late Permian radiolarians are much commoner than Middle Permian radiolarians and Early Permian radiolarians. A review of the taxonomical composition of Late Permian radiolarians of the Belt shows that faunas from different localities can be classed into three groups, according to their contents of albaillellaians, latentifistularians, entactinarians and spumellarians, whose depositional environments correspond respectively to deep water (>500 m), intermediate water (200–500 m), and shallow water depths (<200 m). Such a composition indicates that the Late Permian radiolarian‐bearing siliceous rocks in the Changning‐Menglian Belt are slices from various palaeogeographic settings.
This study investigates the depositional setting of the Oligocene sedimentary rocks of the southern Indus Basin according to grain size analysis of sandstones. Twelve sandstone samples were collected and studied from the Oligocene Nari Formation that distributes along the N‐S‐trending Laki Range in the Kirthar Province of Pakistan. The analysis results specify very fine to fine‐grained sediments and unimodal nature of grain size distribution. The sorting values of the studied sandstones vary from 0.66φ to 0.90φ, which indicates that the sediments are moderately to moderately well‐sorted. The skewness values of investigated samples vary from −0.04φ to 0.25φ, which indicate nearly symmetrical to fine‐skewed nature. The kurtosis of the studied samples varies between 0.73φ and 1.44φ which shows platykurtic, mesokurtic, and leptokurtic nature. Linear discriminant function and interpretation diagrams of sediments demonstrate that the Oligocene Nari Formation sediments from Lal Bagh section were being transported as bedload and suspended load with a bimodal to unimodal depositional mode that were deposited in a fluvial to shallow marine deltaic environment of a passive margin.
The continental arc and intraplate magmatic rocks occur widely within the Altun-Qilian-North Qaidam (AQNQ) region of NW China and play a key role in reconstructing the assembly and dispersal history of micro-continental blocks within a complex orogenic system. In the South Qilian belt, amphibolites from the Hualong Complex can be subdivided into four types based on their lithologies, ages, and tectonic affinities. Type I is characterized by garnet and Proterozoic detrital zircons, indicating a metasedimentary origin. Type II and IV amphibolites are tholeiitic and calc-alkaline, respectively, and characterized by enrichment in LREES, Th, U, and Ba, accompanied by depletion in Nb and Ta, indicating a continental arc setting on an active continental margin. Type III is alkaline with ocean island basalt affinity and considered to have intraplate, rift-related origins. SHRIMP U-Pb dating and previous studies suggest that type II, III, and IV amphiboltes are formed at 1126 Ma, 882 Ma, and 470 Ma, respectively, which corresponds to 1126–895 Ma continental arc magmatism, 882–580 Ma rifted-related magmatism, and 522–440 Ma continental arc magmatism within the AQNQ. The three episodes of magmatism further demonstrate the processes of assembly and breakup of the Proterozoic supercontinent Rodinia and reassembly of the Altun-Qilian-North Qaidam orogenic belt.
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