Northwestern Margin Research Articles

Petrogenesis and tectonic implications of the Neoproterozoic mafic intrusions in the Bikou Terrane along the northwestern margin of the Yangtze Block, South China

The Neoproterozoic mafic intrusions in the Bikou Terrane along the northwestern margin of the Yangtze Block record crucial information on evaluating the tectonic evolution of the Yangtze Block, South China. However, their petrogenesis and tectonic significance are still poorly constrained. In this paper, we present the results of a comprehensive study of zircon U–Pb geochronology and whole-rock geochemistry from the gabbro-diorites and gabbros of the Bikou Terrane. Zircon U–Pb dating results indicate that these gabbro-diorite and gabbro samples were formed coevally at ca. 880 Ma. The gabbro-diorite samples are calc-alkaline in composition and have intermediate contents of SiO2, Mg#, Ni, and Cr. They are enriched in LREEs, and depleted in HFSEs (i.e., Nb, Ta, Zr, and Hf). They have relatively high ratios of Th/Yb, and low ratios of Nb/Yb, Nb/La, Th/Nd, Th/Zr, and U/Th, indicating that they were derived from partial melting of a mantle wedge source metasomatized by slab-derived melts. The gabbro samples are commonly classified as sub-alkaline tholeiitic rocks, and have constantly low to medium values of MgO, Mg#, Cr, and Ni, with distinct spikes at Th, U, Nb and Ta concentrations. They show an E-MORB-like enriched LREEs, low ratios of Nb/La, Ta/Yb, Th/La, and Nb/Zr, and medium ratios of Nb/Yb and Th/Yb, implying that they were most probably originated from the interaction between the upwelling enriched asthenospheric mantle and the mantle wedge peridotite. This mechanism may be triggered by the slab rollback process in a subduction-related environment. Our new investigations, along with available observations from other scholars, support the persistent subduction-related system along the northwestern margin of the Yangtze Block during the early Neoproterozoic.

Zircon and monazite geochronology from the Rengali-Eastern Ghats Province: Implications for the tectonic evolution of the eastern Indian terrane

The tectonic evolution of the Rengali Province and its eventual juxtaposition to the Eastern Ghats Province has important bearings on the geological evolution of the Eastern Indian terrane. New zircon and monazite age data from the Rengali Province and the northern-northwestern part of the Eastern Ghats are presented in this study to trace this evolutionary history. Monazite U-Th-total Pb data from the paragneissic rocks of the eastern Rengali Province show a metamorphic age of 2775 ± 18 Ma while an older age of 2943 ± 35 Ma from the same rock probably suggests an older metamorphic/magmatic event. Zircon U-Pb (LA-ICPMS) data from the northern part of the Eastern Ghats show 1230 ± 21 Ma and 1220 ± 9 Ma ages that we interpret as a major phase of high-grade metamorphism of the basement. Paragneissic rocks from the northwestern margin of the Eastern Ghats yields monazite ages of 966 ± 21 Ma and 555 ± 12 Ma respectively from the core and rim parts of monazite grains. Similar ages of 966 ± 25 Ma and 540 ± 12 Ma are reported from paragneissic rock occurring at the contact of Rengali Province and the Eastern Ghats. This younger (~555–540 Ma) age likely correlates to the amphibolite facies reworking of the granulitic lower crust which coincides with the emplacement of nepheline syenite at 556 ± 28 Ma (zircon U-Pb data) and the contact metamorphism of the ultramafic granofels at 553 ± 18 Ma (monazite data). Nepheline monzosyenite veins intruded the gneissic nepheline syenite at 506 ± 9 Ma (zircon U-Pb data). Emplacement of the monzosyenite veins within the felsic gneiss country at the northwestern margin of the Eastern Ghats at 490 ± 3 Ma (zircon U-Pb data) marks the last thermal imprint in response to large-scale shear-induced deformation at the northern/northwestern contacts of the Eastern Ghats. We infer that the Neoarchean (ca. 2943–2775 Ma) events possibly resulted from the ensuing convergent tectonics driven by lithospheric peeling (peel-back convergent tectonics). The Eastern Ghats and its Antarctic counterpart juxtaposed with the Rengali Province during ca. 1000–900 Ma and become a part of the Eastern Indian terrane. The Ediacaran-Cambrian (ca. 556–490 Ma) events imply the reactivation of the deep crustal Tonian-age shear systems in a transpressional tectonic setting.

Structural analysis of the Wichita Uplift and structures in the Anadarko Basin, Southern Oklahoma

We have constructed regional structural transects across the Wichita Uplift and the adjacent Anadarko Basin to show the relationship between thick-skinned basement-involved structures and thin-skinned detached fold-thrust structures. Slip from the basement-involved structures in the Wichita Uplift is transferred along two major detachments into the Anadarko Basin. Our interpretation is that along the northwestern margin, the Wichita Uplift is marked by a zone of frontal imbricates forming a triangular wedge with most of the slip dissipated along the Wichita front. Paleozoic units show tight folding with overturned beds in the frontal zone. The uplift is episodic as indicated by the truncation of major faults along unconformities and their subsequent reactivation. In contrast, along the southeast margin, a significant part of the slip is transferred into structures in the Anadarko Basin. These structures are tight faulted-detachment folds that formed above a major detachment within the Springer Shale, cored by broader structures detaching at the base of the Arbuckle Group. Examples include the Carter-Knox, Cement-Chickasha, and Cruce structures. Oblique faults with normal and strike-slip components cut some of these structures, resulting in more complex geometries. We have determined that preexisting normal faults of Precambrian-Cambrian age were either reactivated along the Wichita Uplift or they controlled the location of the Pennsylvanian-age structures in the Anadarko Basin. Progressive rotation of regional stresses from northeast–southwest to a more east-northeast–west-southwest direction during the Pennsylvanian impacted the tectonic history of the area. We used 2D and 3D seismic, well-log data, and surface geology to evaluate the structural styles and tectonic evolution of the Wichita Uplift and the Anadarko Basin.

Discovery of Permian crustal carbonatite in the northwestern margin of Tarim Basin and its geological significance

碳酸岩是自然界较为特殊的一类火成岩，由于其在上侵过程中不易受到地壳物质的混染，可以很好地保留源区原始特征，因而在揭示岩浆源区属性和地球深部碳循环等方面具有重要的研究价值。塔里木盆地西北缘阿克苏-乌什南部地区发育多条侵位到新元古界南华系中的火成碳酸岩岩墙或岩脉。这些岩脉的锆石U-Pb测年结果显示出多个年龄峰值，其中最年轻一组（3颗）岩浆锆石的谐和加权年龄为272±4Ma，另外还有400Ma、450Ma、790Ma、850Ma等年龄峰，而这些锆石可能均为碳酸岩岩浆捕获成因。结合区域上发育大量早二叠世辉绿岩岩床及岩墙等侵入事件，推测这些碳酸岩很可能形成于早二叠世（~270Ma）。碳酸岩主要由方解石（90%以上）构成，并含少量的白云石、重晶石、天青石、赤铁矿等其它矿物，可见方解石与白云石之间明显的出溶结构以及野外宏观上碳酸岩包裹围岩等现象。元素地球化学分析显示，碳酸岩CaO含量高（44.40%~50.40%），SiO₂含量低（1.83%~7.97%）；稀土元素总量很低（ΣREE=21.67×10^-6~91.32×10^-6），轻重稀土分异明显（LREE/HREE=3.90~8.40），具有中等负铕异常（δEu=0.61~0.68）和中等至弱负铈异常（δCe=0.59~0.98）；大离子亲石元素Sr、Rb、Ba等相对富集，Nb、Ta、Ti等高场强元素相对亏损；稳定同位素δ¹³C_V-PDB值高（-2.69‰~-2.93‰）、δ¹⁸O_V-SMOW值高（17.33‰~17.86‰）。这些特征与壳源碳酸岩特征相似，而与幔源碳酸岩差别较大。Sr-Nd同位素结果显示高的（⁸⁷Sr/⁸⁶Sr）_i值（0.710106~0.710558），低的ε_Nd（t）值（-8.46~-12.80），也进一步证实碳酸岩为地壳来源。综合分析认为这些碳酸岩很可能是塔里木二叠纪大火成岩省晚期岩浆事件的组成部分，系由塔里木西北缘地幔柱减压熔融后形成高温的基性岩浆向上侵位运移时，导致下地壳的碳酸盐储库部分熔融形成碳酸岩岩浆，并沿着断裂快速向上侵位形成。本文研究不仅丰富了塔里木大火成岩省的岩石组成认识，而且有助于增进对二叠纪深部复杂壳幔作用过程的理解。;Igneous carbonatites is a kind of special igneous rock in nature. Because they are not easily contaminated by crustal materials in the process of upwelling, they can well retain the original characteristics of the magma source. Therefore, they have important research value in revealing the properties of the magma source and the carbon cycle of the deep earth. Several carbonatite dikes or dykes are found intruded into the Neoproterozoic Nanhua System in the southern Aksu-Wushi area, the northwestern margin of Tarim Basin. Zircon U-Pb geochronology dating of these carbonatites yield multiple peak ages, including a youngest one at 272±4Ma obtained by three zircons, and other ones at 400Ma, 450Ma, 790Ma, 850Ma, respectively. However, all these zircons were probably captured by carbonate magma. Considering the intrusive events of massive Early Permian diabase sills and dikes in the region, we speculate that these carbonatite dikes were probably formed in the Early Permian (~270Ma). The carbonatites are mainly composed of calcite (over 90%) and a small amount of dolomite, barite, celestite, hematite and other minerals. In addition, the carbonatites have obvious exsolution structures between calcite and dolomite, and the country rock breccias are found wrapped by the carbonatites in the field. The element geochemistry analysis results show that carbonatites have high CaO content (44.40%~50.40%), low SiO₂ content (1.83%~7.97%), extremely low total rare earth elements content (21.67×10^-6~91.32×10^-6) with obvious differentiation of LREE and HREE (LREE/HREE=3.90~8.40) and weak negative Eu and Ce anomalies (δEu=0.61~0.68; δCe=0.59~0.98). And the large ion lithophile elements like Sr, Rb, Ba are relatively enriched, and high field-strength elements such as Nb, Ta, Ti are relatively depleted. The stable isotope of δ¹³C_V-PDB and δ¹⁸O_V-SMOW are high (-2.69~-2.93‰ and 17.33~17.86‰, respectively). All these characteristics are similar to the crustal carbonatite, but quite different from the mantle carbonatite. The Sr-Nd isotopic results of high (⁸⁷Sr/⁸⁶Sr)_i values (0.710106~0.710558) and low ε_Nd(t) values (-8.46~-12.80) also confirm that the Aksu carbonatites are derived from the crust. After comprehensive analysis, we propose that these carbonatites are probably the components of the late magmatic event in the Permian Tarim Large Igneous Province. When the mantle plume in the northwestern margin of Tarim was decompressed, it would form high-temperature basic magma and emplaced upward. These basic magma would heated the carbonate reservoir in the lower crust and partially melted it to form carbonatites magma, which was rapidly emplaced upward along the extension fault. This study not only enriched the petrological compositions of the Permian Tarim Large Igneous Province, but also promoted the understanding of the complex crust-mantle interaction processes in the deep area of Permian Tarim Basin.

Geochemical and geochronological constraints on the genesis of Pliocene post-collisional granite porphyry and shoshonite in Quanshuigou, western Kunlun Mountains, NW Qinghai–Tibet Plateau

ABSTRACT Post-collisional K-rich magmatic rocks of the Qinghai–Tibet Plateau provide important information for understanding the continental collision and uplift of this plateau during the Cenozoic. However, the source of these K-rich rocks is still debated, and limited investigations have been conducted in the western Kunlun Mountains (WKM) in the northwestern margin of the plateau. In this paper, we present geochemical, geochronological, and Sr–Nd–Hf isotope data for Quanshuigou post-collisional K-rich igneous rocks from the WKM. Geochronological results show that these rocks were emplaced during two stages: (1) ca. 5.3 Ma, as an intrusive suite of granite porphyries; and (2) ca. 4.8 Ma, as a suite of shoshonites. These rocks are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and depleted in heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs). Compositions of the granite porphyries classify these rocks as high-K calc-alkaline peraluminous A-type granites. Combining the negative zircon ε Hf(t) values (−6.9 to −1.1) with Hf crustal model ages (TDM2) of 1.16–1.53 Ga and negative ε Nd(t) values (−5.31 to −4.96) with two-stage Nd model ages (TDM2) of 1.22–1.46 Ga, these signatures suggest that the granite porphyries most probably originated from partial melting of thickened lower crust with limited input of mantle-derived magmas. In contrast, the shoshonites have low SiO2 contents (50.84–53.94 wt.%), relatively high Mg# values (50–57), and εNd (t) values ranging from −6.18 to −5.34, with single-stage Nd model ages (TDM1) of 0.97–1.02 Ga. Our analyses show that the parental magma of the shoshonites probably formed by partial melting of EMII-type lithospheric mantle, with the addition of subducted and recycled oceanic sediments. Hence, we conclude that these granite porphyries and shoshonites formed in a post-collisional, extension-related geodynamic setting and that the melting was probably triggered by motion on the Altyn Tagh strike-slip fault system.

Sedimentary structure of the Sichuan Basin derived from seismic ambient noise tomography

SUMMARY Determining a detailed 3-D velocity model with high resolution for the sedimentary layer in the Sichuan Basin is potentially beneficial both to the industrial oil/gas exploration and earthquake hazards’ mitigation. In this study, we apply the ambient noise tomography method to construct a 3-D S-wave velocity model. This model focuses on the sedimentary layer of the Sichuan Basin, with a 0.3° × 0.3° grid precision. Dispersion curves of both group and phase velocities of Rayleigh wave at 4 to 40 s periods are utilized, which are extracted from 87 broad-band stations in the Sichuan Basin and the surrounding areas. The 3-D model reveals a thick sedimentary layer of the Sichuan Basin with S-wave velocity ranging from ∼2.0 to 3.4 km s−1. The sediment thickness in the margins of the Sichuan Basin is generally greater than the typical values of 6–10 km in the central areas due to surrounding orogenic activities, with a maximum depth of ∼13 km in the northwestern margin. Moreover, a prominent low S-wave velocity anomaly in the margins may be caused by the sediment accumulations from large-scale landslides and pronounced denudation of the surrounding orogenic belts. Major geologic units in the sedimentary layer are delineated in this study. The S-wave velocity values within each geologic unit and their bottom interfaces are obtained. Based on our model, we calculate synthetic ground motions for the 2013 Lushan earthquake and obtain the distribution of the peak ground acceleration from the earthquake epicentre to the western Sichuan Basin. The result clearly illustrates the basin amplification effect on the seismic waves.

Distribution features and main controlling factors of volcanic buried hill reservoirs in carboniferous basement of Junggar Basin

On the northwestern margin of Junggar Basin, the complex volcanic oil reservoirs in the Carboniferous basement are highly concealed, poor physical properties, and difficult to explore or characterize. With the purpose of understanding the distribution of favorable reservoirs, the geological features, distribution law, and control factors of the reservoirs are fully explored based on analysis of the data from coring, logging, seismic analysis, and thin section in this paper. And some conclusions can be drawn as follows: (1) the Carboniferous rocks in the study area mainly include tuff lava, tuff, sedimentary tuff, tuffaceous sandstone, and tuffaceous glutenite; (3) the reservoirs have low porosity and permeability, the secondary pores and fractures provide the main storage spaces, the reservoirs fall into fractured reservoir and pored-fractured reservoir; (3) the favorable weathered crust reservoirs mainly exist in strong and moderately weathered zones in the vertical direction, which are 20–130 m from the top of the Carboniferous basement; in the planar direction, the reservoirs mostly concentrate around J353-J354-J336-J399-J132-J126 wells and around J12-J329 wells; (4) the reservoirs are mainly controlled by weathering and leaching, and the storage capacity is greatly improved by tectonic movement. This paper discloses the main controlling factors and distribution features of high-quality volcanic reservoirs in the Carboniferous basement on the northwestern margin of Junggar Basin, laying a solid basis and guidance for the exploration and development of similar oil and gas reservoirs.

Tectonic Conditions for the Formation of Structures of the Sakandzha Ore District (Selennyakh Ridge, NE Asia)

The Sakandzha gold-ore district occurs in the north-western margin of the Selennyakh allochthonous block of the Omulevka terrane forming part of the Kolyma-Omolon micricontinent (Verkhoyansk-Chersky orogenic belt, NE Asia). Tectonic structures of the Sakandzha ore district were formed in two stages of the Late Mesozoic deformation: thrust and strike-slip fault ones. Formation of the fold-and-thrust structures of the early Late Mesozoic deformation stage within the Selennyakh block and its thrusting, in the form of a thrust sheet, over the fold structures of the Tuostakh-Polousnyi zone in Neocomian time were almost synchronous with the intrusion of large granite plutons (Sakhanya and Syachan) into the fold-and-thrust structures in the northern part of the Verkhoyansk-Chersky orogenic belt. By the end of the Neocomian or in pre-Aptian time, strike-slip deformations of the second Late Mesozoic stage were superposed on the early thrust structures. At that time, the Selennyakh block was divided into smaller blocks by strike-slip faults, among them the Kalychan one. Two ore-bearing segments of different structure, Pologiy and Arbat, are distinguished within the Kalychan dextral strike-slip fault zone. As a result of strike-slip motions, the sedimentary rocks of these segments became steeply inclined, strongly deformed, and were pierced by a network of variably oriented fractures and cracks, which made them highly permeable for the circulating hydrothermal and ore solutions. The major Kalychan, Upper Kalychan and Arbat strike-slip faults are interpreted as the ore-feeding faults. The feathering subsidiary faults as well as strongly fractured zones are considered as ore-controlling and ore-bearing structures. The end of the Neocomian was marked by completion of the collision, and the Aptian-Albian period saw the inception of the Dzhakhtardakh volcanogenic belt of Early to Late Cretaceous age. Its formation is considered to be related to the crustal extension processes preceding the opening of the Eurasian ocean basin.

Ultramafic xenoliths from aillikites in the Tarim large igneous province: Implications for Alaskan-type affinity and role of subduction

The Tarim large igneous province (TLIP) located in northwestern China is characterized by a relatively long duration of magmatism and complex lithology as compared to other global LIPs. Previous studies correlated these features to the interaction between mantle plume and subducted slab. However, no direct subduction-related magmatic records were so far recognized to support this interpretation. Here we report the petrological, mineralogical, and geochemical characteristics of a suite of ultramafic xenoliths occurring within aillikites from the northwestern margin of the TLIP. The xenoliths include dunite, wehrlite, olivine clinopyroxenite, clinopyroxenite, amphibole clinopyroxenite, clinopyroxene hornblendite, and hornblendite. These rocks show broadly similar mineralogy and are mainly composed of olivine, clinopyroxene and amphibole with rare phlogopite, chromite, and magnetite, in the absence of orthopyroxene and plagioclase. We evaluate the petrogenesis of these rocks based on whole-rock major and trace element compositions and mineral chemistry. The rocks exhibit enrichment of large ion lithophile elements (LILEs) relative to high field strength elements (HFSE) with negative Nb, Ta, and Ti anomalies, similar to the features of arc-like lavas. Their (87Sr/86Sr)t (0.7035–0.7041) and εNd(t) (1.17–5.67) values are consistent with depleted mantle source. The Mg# [molar Mg/(Mg + Fe2+)] and Fe3+# [molar Fe3+/(Fe3++Al + Cr)] values of spinel in the ultramafic xenoliths are consistent with those of the Alaskan-type mafic-ultramafic intrusions. The Al2O3 contents in clinopyroxene increase with decreasing Mg#, which is comparable with the Al2O3 variation trend of clinopyroxene in Alaskan-type and arc-related magma, in contrast to those of the LIPs. Our data establish that the ultramafic xenoliths in the aillikites are comparable to Alaskan-type ultramafic complexes, and correspond to magmatism associated with the southward subduction of oceanic plate beneath the Tarim craton during Neoproterozoic or Early Paleozoic. Our study provides direct evidence for the interaction between mantle plume and subducted slab in generating the TLIP.

The accretion history of the South China Block at its northwest margin in the Neoproterozoic: Records from the Changba complex in the Mianlue zone

To constrain the accretion history of the South China Block (SCB), a geochronological and geochemical study was carried out for the Changba Complex within the Mianlue zone in the northwestern margin of the SCB. Zircon U-Pb dating reveals that the metagabbros and metatonalites were formed at ca. 985–950 Ma, while the bt-amp gneisses have a younger protolith age of ca. 830 Ma. The metagabbro and metatonalite are low- to medium-K tholeiitic series and are significantly enriched in LILEs (Rb, Ba, Th, U, K) and slightly enriched in LREEs, but depleted in HFSEs such as Nb, Ta and Ti. In addition, they have highly positive zircon εHf(t) values (+11.3~ +16.9) and whole-rock εNd(t) values (+3.9~ +5.5) and εHf(t) values (+11.0 ~ +11.8). These geochemical features resemble to those of the magmatic rocks in juvenile oceanic arcs. Zircon δ18O value ranges from 3.0 to 6.2‰ for metagabbro and 4.6 to 5.34‰ for metatonalite, which are slightly lower than the mantle values. Hydrothermally altered gabbros in the lower oceanic crust often have lower δ18O values than mantle and can be primary components of arc lower crust. Therefore the Changba tonalite is interpreted to have been formed by partial melting of the hydrothermally altered gabbroic lower crust. The ca. 830 Ma gneisses are enriched in LREEs and incompatible elements and depleted in HFSEs, similar to the trace element pattern of the average bulk continental crust. They have enriched Sr-Nd-Hf isotopic compositions (0.7055 ~ 0.7060, −5.1 ~ −6.3, and −8.5 ~ −8.8, respectively), which is distinct from the juvenile arc crust. Besides, variable zircon εHf(t) values of −40.6 to 0 and slightly low to elevated zircon δ18O values of 4.2 to 6.6‰, suggesting derivation from reworking of a heterogeneous crustal source. In this regard, the Changba complex records the accretion of juvenile island arc and subsequent reworking of both juvenile and ancient crustal materials, which highlights the crustal growth and geodynamical evolution of the northern margin of the South China Block during the Neoproterozoic in response to the assembly of Rodinia.

Late Triassic rift tectonics at the northernmost Andean margin (Sierra Nevada de Santa Marta)

Break-up of western Pangaea initiated a back-arc type tectonic setting along the northwestern margin of Gondwana, which resulted in the accumulation of sedimentary and volcanic deposits in extensional basins during the Middle-Late Triassic. The Sierra Nevada de Santa Marta massif, at the northwestern termination of the Andes, exposes Upper Triassic syn-rift successions beneath a voluminous Lower Jurassic volcanic cover. In this contribution, we present a sedimentological and stratigraphical analysis of the Upper Triassic Los Indios and Corual formations, by integrating whole-rock geochemistry, zircon U–Pb geochronology, and a biostratigraphic revaluation of the conchostracan fossil content. We divide the Los Indios Formation into two sandy to conglomeratic and two muddy members. First and third members were deposited in a coarse-grained delta environment. The second and fourth members are interpreted to represent offshore deposits resulting from the encroachment of gravity-flow lobes. Volcanoclastic and volcanic rocks interbedded with sedimentary deposits of the Los Indios and Corual formations record bimodal basic-acid volcanism with a variably alkaline tendency. Based on their alkaline and calc-alkaline signature, and rare earth and trace element patterns, volcanic rocks display a back-arc affinity. With respect to the age of the Los Indios Formation, we recovered conchostracans diagnostic of Upper Carnian and Lower Norian fauna, bearing similarity with faunal assemblages of other Upper Triassic deposits exposed in the northern Andes. The Los Indios and Corual formations are considered as syn-rift successions in a back-arc extension setting. The development of this back-arc basin coincided with the Late Triassic separation of the Oaxaquia, Acatlán, and Chortis terranes during Pangaea break-up. Finally, the Early to Middle Jurassic evolution of the NW Gondwana margin is marked by the inception of a magmatic arc, which characterizes the establishment of Andean-type subduction coeval to ignimbrite flare-up.

Mesophilous Quercus frainetto Dominated Forests from Western Balkans

Hungarian oak (Quercus frainetto) has dominant or co-dominant role in many thermophilous deciduous forest communities in the Balkan Peninsula. However, recent field research in the north-western margin of its range has revealed that some stands have a pronounced mesophilous character, which was also supported by data from the literature. This paper aims to analyze this mesophilous community of Hungarian oak which is found in north-western Serbia, north-eastern Bosnia and Herzegovina (B&H) and eastern Croatia. Numerical analysis, which included classification and ordination of all 474 available relevés of Hungarian oak forests from the western and central Balkans, along with the 43 relevés of mesophilous forests of oak-hornbeam from B&H, have shown that 83 mesophilous Hungarian oak relevés are floristically and ecologically more similar to mesophilous forests of sessile oak-hornbeam (Erythronio-Carpinion) than to xero-thermophilous forests of Qeurcion confertae. The new association Carpino betuli-Quercetum frainetto ass. nova hoc loco was described, floristically and ecologically characterized and assigned to mesophilous oak-hornbeam forests of Erythronio-Carpinion. The study also discusses the syntaxonomical issues of Quercion confertae and its central association Quercetum frainetto-cerridis, considering the problem of the lack of good diagnostic species for both syntaxa, and introduces possible ways of dealing with these issues.

A review on Ordovician conodont biostratigraphy of the North China Plate and new research advances on its northwestern margin

A review on Ordovician conodont biostratigraphy of the North China Plate and new research advances on its northwestern margin

Quantitative assessment of the relative tectonic activity using the analytical hierarchy process in the northwestern margin of the Lut Block, Central Iran

In this study, the analytical hierarchy process (AHP) was used to enhance the accuracy of the results obtained from the geomorphic analysis of tectonic activity. For this purpose, the relative tectonic activity index was computed by applying different and specific weights to each index based on the general geological and structural conditions of the study area. The Kuh-e-Sarhangi Mountain with a NE-SW strike is located in the northern part of the Tabas Block in the Central Iran Plate. The Kuh-e-Sarhangi Fault is considered one of the major faults in Central Iran and shows evidence of Quaternary activity. In this study, in order to evaluate the activity of the faults, six morphotectonic indices were computed and the different conditions which influenced the morphometric factors were considered and analyzed. The relative tectonic activity of Kuh-e-Sarhangi was classified based on the analytical hierarchy process method. These results were confirmed by field studies. The results of this method fitted well with some independently detected Quaternary faults and other reactivated faults in the Kuh-e-Sarhangi Mountain. With a right-lateral movement, some of these faults have caused the displacement of rivers up to 900 m. The dispersal of the high tectonic activity classes based on the AHP method conforms to the post-Neogene activity of the region. The distribution of the classes illustrates that morphotectonic anomalies correspond with the incised alluvial fans and terraces and the bisected and displaced rivers in the study area. Accordingly, the integration of the results of field investigations, satellite images, morphometry, and the AHP method shows a high accordance with the active structures and demonstrates that the study area is tectonically active. This analysis illustrates the recent movements of the Kuh-e-Sarhangi Fault and the activity of the Lut Block due to the convergent movement of the Arabian plate toward Eurasia.

Post-seismic deformation mechanism of the July 2015\xa0MW 6.5 Pishan earthquake revealed by Sentinel-1A InSAR observation

On 3 July 2015, the Mw 6.5 Pishan earthquake occurred at the junction of the southwestern margin of the Tarim Basin and the northwestern margin of the Tibetan Plateau. To understand the seismogenic mechanism and the post-seismic deformation behavior, we investigated the characteristics of the post-seismic deformation fields in the seismic area, using 9 Sentinel-1A TOPS synthetic aperture radar (SAR) images acquired from 18 July 2015 to 22 September 2016 with the Small Baseline Subset Interferometric SAR (SBAS-InSAR) technique. Postseismic LOS deformation displayed logarithmic behavior, and the temporal evolution of the post-seismic deformation is consistent with the aftershock sequence. The main driving mechanism of near-field post-seismic displacement was most likely to be afterslip on the fault and the entire creep process consists of three creeping stages. Afterward, we used the steepest descent method to invert the afterslip evolution process and analyzed the relationship between post-seismic afterslip and co-seismic slip. The results witness that 447 days after the mainshock (22 September 2016), the afterslip was concentrated within one principal slip center. It was located 5–25 km along the fault strike, 0–10 km along with the fault dip, with a cumulative peak slip of 0.18 m. The 447 days afterslip seismic moment was approximately 2.65 × 1017 N m, accounting for approximately 4.1% of the co-seismic geodetic moment. The deep afterslip revealed that a creeping process from steady-state “secondary” creeping to accelerating “tertiary” creep in the deep of fault. The future seismic hazard deserves further attention and research.

Fragmentation of South China from greater India during the Rodinia-Gondwana transition

AbstractLate Tonian to Cambrian sedimentary sequences in northwestern India and South China provide vital evidence for modeling their paleogeographic linkage, including their juxtaposition and subsequent separation during the transition from the Rodinia to the Gondwana supercontinents. Similarities in lithostratigraphy and detrital zircon U-Pb-Hf-O isotopic characteristics in the late Tonian sedimentary units from both regions underline a common provenance. A substantial decrease in zircon δ18O values from super- to sub-mantle compositions and simultaneous increase in the zircon εHf(t) values in South China and northwestern India for the 800–700 Ma time window suggest a common Neoproterozoic extensional magmatic event, corresponding with the Rodinia breakup. A distinct change in sedimentary provenance is noted during the Cryogenian period. Sedimentation along the northwestern margin of India for the remainder of the Neoproterozoic encompasses large volumes of clastic detritus dominated by old zircon ages, derived inboard from the Indian craton. In contrast, contemporaneous sedimentary units in the Yangtze region of South China are dominated by Neoproterozoic zircons. The detrital zircon age data underline a close paleogeographic linkage between northwestern India and South China (Yangtze and Cathaysia regions) in the Rodinia supercontinent configuration and argue for their separation through continental rifting during the Cryogenian. Northwestern India developed into a passive margin, whereas the South China block partially rifted, rotated, and migrated dextrally along the Gondwana margin toward northeastern India and Western Australia, such that the Cathaysia block continued to receive detritus from Gondwana continental regions.

The first record of nautiloids (Cenoceras intermedius) from the early Pliensbachian of Rgotina, (Getic nappe, eastern Serbia)

ABSTRACT Cenoceras intermedius (J. Sowerby) is one of the most widespread species among the Early Jurassic nautiloid fauna and its geological range still controversial up to the present day. The specimen of Cenoceras intermedius from the sandy bioclastic limestones of Rgotina (Getic nappe of eastern Serbia), is a large half conch representing the phragmocone. Viewing our findings in light of previous results suggests the main morphological changes occurred during the ontogenetic evolution. The coiling starts with a round-square whorl section, to become almost quadrangular (penultimate whorl) and trapezoidal-triangular for the last one. Moreover, the siphuncle foramen moves within the septum towards the venter border from a slight upper position against the centre in the inner whorls. It exhibits a typical suture line for the genus Cenoceras with two saddles and three lobes. This finding certainly extends the geological range of the species to after the Sinemurian contrary to several earlier works that confined it only to this stage. This assertion is based on the accurate assignment of the nautiloid-bearing bed of Rgotina to the early Pliensbachian by the index species Uptonia jamesoni. The Serbian specimen supports the species’ palaeobiogeographic widespread range within the north-western margin of the Neo-Tethys Ocean framework.

Fault inversion in central Iran: Evidence of post Pliocene intracontinental left lateral kinematics at the northern Iranian Plateau margin

Structural inversion of strike–slip faults in response to plate kinematic changes and interactions of large–scale basement blocks is a significant phenomenon in continental collision zones. In the Alborz Mountains, at the northern margin of the Iranian Plateau, Late Cenozoic kinematic of major fault systems changed in the late Pliocene from dextral to sinistral strike–slip. This kinematic change has been attributed to the clockwise rotation of the rigid South Caspian Block. However, the spatial extent and distribution of this kinematic change toward the south and into the Iranian Plateau is controversial. Here we present a detailed structural analysis of the West Saveh fault system located at the northwestern margin of the Iranian Plateau to unravel the regional distribution of this kinematic change. The West Saveh fault system is composed of the WNW–ESE trending Kushk–e Nosrat, Khalkhab, Saveh, and Nashveh faults. We identified several different sets of strike–slip related structures, which are classified into three categories based on their cross–cutting relationship and superimposition of kinematic indicators. We document dextral, dextral transpression and sinistral kinematics and their relative timing since Miocene time in the West Saveh fault system. Our structural analysis indicates recent slip sense inversion from dextral transpression to sinistral strike–slip, which possibly occurred in post Pliocene time. This kinematic change is regional and can be explained by the clockwise rotation of the South Caspian Block similarly to the kinematic change documented in the Alborz Mountains. We propose that the kinematic change due to South Caspian Block rotation is not restricted to the Alborz Mountains, but also affected the Iranian Plateau during post Pliocene time.

Contributions to the knowledge of plutonism of the Mocoa-Santa Marta Arc during the Early - Middle Jurassic, on the northwestern margin of the Andes, Colombia

A partir de la integración de información de campo, con datos petrográficos, geoquímicos de roca total, isotópicos y geocronológicos, se realizó la comparación de los diferentes segmentos que conforman el Arco Mocoa-Santa Marta (AMSM). Este arco, que aflora en el Valle Superior del Magdalena, la Serranía de San Lucas, la Sierra Nevada de Santa Marta y la Alta Guajira, se emplazó en un basamento Neo-proterozoico y en granitoides de un arco Pérmico, los cuales fueron posteriormente dispersados a lo largo de la Paleomargen Suramericana. El plutonismo del AMSM se caracteriza por la migración composicional en sentido W-E, con la formación de intrusivos de cuarzomonzonita, monzonita, charnoquita, tonalita y granodiorita hacia el occidente y la formación de plutones de monzogranito y sienogranito hacia el oriente; también existe un enriquecimiento en SiO2 en sentido S-N. Los plutones del AMSM cristalizaron entre el Jurásico Temprano (~193 Ma) y el Jurásico Medio (~164 Ma), y fueron generados por al menos cuatro eventos de cristalización. Los plutones que conforman este arco son de Tipo I, calcoalcalinos con leve variación hacia los granitoides alcalino cálcicos. Los plutones más antiguos y occidentales son mayoritariamente metaluminosos y los más jóvenes y orientales son peraluminosos, todos ellos formados dentro del mismo contexto de evolución magmática de un arco de margen continental erosiva con vulcanismo asociado. La temperatura de cristalización del circón TzircTi para el conjunto de plutones del Cinturón Occidental presenta un pico de cristalización a 860oC, mientras que los plutones del Cinturón Oriental presentan picos de cristalización a 714oC, 807oC y 854oC, con una alta dispersión.

Gold and antimony metallogenic relations and ore-forming process of Qinglong Sb(Au) deposit in Youjiang basin, SW China: Sulfide trace elements and sulfur isotopes

In the northwestern margin of the Youjiang basin (NWYB) in SW China, many Carlin-like gold deposits are highly antimony (Sb)-rich, and many vein-type Sb deposits contain much Au. These deposits have similar ages, host rocks, ore-forming temperatures, ore-related alterations and ore mineral assemblages, but the Au and Sb metallogenic relations and their ore-forming process remain enigmatic. Here we investigate the large Qinglong Sb deposit in the NWYB, which has extensive sub-economic Au mineralization, and present a new metallogenic model based on in-situ trace elements (EPMA and LA-ICP-MS) and sulfur isotopes (NanoSIMS and fs-LA-MC-ICP-MS) of the ore sulfides. At Qinglong, economic Sb ores contain coarse-grained stibnite, jasperoid quartz and fluorite, whilst the sub-economic Au–Sb ores comprise dominantly veined quartz, arsenian pyrite and fine-grained stibnite. Three generations of ore-related pyrite (Py1, Py2 and Py3) and two generations of stibnite (Stb1 and Stb2) are identified based on their texture, chemistry, and sulfur isotopes. The pre-ore Py1 is characterized by the lower ore element (Au, As, Sb, Cu and Ag) contents (mostly below the LA-ICP-MS detection limit) and Co/Ni ratios (average 0.31) than the ore-stage pyrites (Py2 and Py3), implying a sedimentary/diagenetic origin. The Py2 and Py3 have elevated ore element abundance (maximum As ​= ​6500 ​ppm, Au ​= ​22 ​ppm, Sb ​= ​6300 ​ppm, Cu ​= ​951 ​ppm, Ag ​= ​77 ​ppm) and Co/Ni ratios (average 1.84), and have positive As vs. Au–Sb–Cu–Ag correlations. Early-ore Stb1 has lower As (0.12–0.30 ​wt.%) than late-ore Stb2 (0.91–1.20 ​wt.%). These features show that the progressive As enrichment in ore sulfides is accompanied by increasing Au, Sb, Cu and Ag with the hydrothermal evolution, thereby making As a good proxy for Au. As-rich, As-poor and As-free zones are identified via NanoSIMS mapping of the Au-bearing pyrite. The As-rich zones in the Qinglong Au-bearing pyrites (Py2 and Py3) and ore stibnites (Stb1 and Stb2) have narrow δ34SH2S ranges (−8.9‰ to +4.1‰, average −3.1‰) and −2.9‰ to +6.9‰, average ​+ ​1.3‰), respectively, indicating that the Au-rich and Sb-rich fluids may have had the same sulfur source. Published in-situ sulfur isotopic data of pyrite As-rich zones from other Carlin-like Au deposits (Shuiyindong, Taipingdong, Nayang, Getang and Lianhuashan) in the NWYB have similar ore-fluid δSH2S34 values (−4.5‰ to +6.7‰, average −0.6‰) to those of Qinglong. Therefore, we infer that the sulfur of both Au and Sb mineralization was derived from the same magmatic-related source (0 ​± ​5‰) in the NWYB.Moreover, the core of pyrites (Py1) has variable S isotope fractionation (−18.9‰ to +18.1‰, mostly +3‰ to +12‰), suggesting that the higher-34S H2S was produced by bacterial sulfate reduction (BSR). The hydrothermal pyrite (Py2 and Py3) δ34S values gradually decrease with increasing As concentrations, and ultimately, within the restricted range (−5‰ to +5‰) in As-rich zones. This variation implies that the As-rich pyrite was formed through ongoing interactions of the magmatic-hydrothermal fluid with pre-existing sedimentary pyrites, causing the progressive decreasing δ34S values with As content increase, Hence, the fluid/mineral interaction may have generated the observed variation in δ34S and As contents. Overall, comparing the Au and Sb deposits in the NWYB, we favor a magmatic-related source for the Au–Sb–As-rich fluids, but the Au- and Sb-ore fluids were likely evolved at separate stages in the ore-forming system.