Late Precambrian Research Articles

Geological position of the Junggar terrane (Southern Kazakhstan) in the structure of Rodinia supercontinent: results of research of the late-precambrian metasedimentary complexes

The results of studying of the Precambrian metasedimentary sequences of the Junggar terrane located in South Kazakhstan are given in the article. In the structure of the Junggar terrane, we studied the rocks of the Sarychebyn group and Kosagash formation. Petrogeochemical data combined with the results of U‒Pb and Lu‒Hf isotope-geochronological studies of the detrital zircons showed that the Sarychebyn Group and the Kosagash formation represent a similar stratigraphic level that accumulation occurred in the Late Mesoproterozoic to Early Neoproterozoic (~1026‒~920 Ma). The main sources of the detrital zircon age populations were the Mesoproterozoic and Paleoproterozoic complexes. Among these complexes can be identified metabasites and metapelites of intermediate and high metamorphic grades, as well as felsic igneous rocks formed with the participation of various sources, can be distinguished. The Junggar terrane exhibits a close tectonic affinity with the Aktau-Mointy, Yili, Issyk-Kul, Chinese Central Tien-Shan, and the Northern Kazakhstan terranes in the Late Precambrian. They were probably located near the Sveconorwegian orogen in the western Baltica within the Rodinia supercontinent structure.

The Ediacara Biota of the Wood Canyon formation: Latest Precambrian macrofossils and sedimentary structures from the southern Great Basin

Fossils of the Ediacara Biota preserve the oldest indisputable evidence of macroscopic animal life. Although new finds continue to expand the near global distribution of such organisms, relatively abundant and diverse populations of these primarily soft-bodied taxa are known from just a handful of localities. More commonly, reports of late Precambrian fossils consist of comparably lower diversity assemblages (less than ∼10 genera), with a limited number of total specimens (less than ∼1000 reported macrofossils over a broad geographic area). Determining the factors responsible for such differences can help to fully appreciate preserved patterns of Ediacaran biodiversity.The lower member of the Wood Canyon Formation in the Death Valley region of the southern Great Basin, USA contains fossils of the Ediacara Biota. Here we redescribe previously recovered specimens based on new data from 3D surface scans and report several new finds from this unit. Although this allows the addition of two Ediacara taxa (Pteridinium and Charnia?) to the known diversity in the region, most potential macrofauna consist of amorphous, irregular forms with minimal preserved structures wherein an abiotic origin cannot be ruled out. A combination of factors – including variable taphonomic conditions, difficulty of identification and an original depauperate community – contribute to the observed diversity of this assemblage. Many biases that impact this record are either common (e.g., age, preservation requirements, difficulty of recognition) or comparable (e.g., paleoenvironmental and taphonomic controls) to known occurrences of Ediacara macrofauana. Thus, we argue this fossil record at least partially reflects the original community composition, suggesting that fossils of the lower Wood Canyon record a signal of relatively low diversity of the Ediacara Biota in the lead up to the Ediacaran-Cambrian boundary.

The Proterozoic–Cambrian salt detachment zone at Lakhar Kuh, Iran: a seismic-scale analogue for salt stringers, detachment folding and multiphase structural development

Abstract In the Middle East, significant evaporite units formed in the latest Precambrian–Cambrian, Triassic, Jurassic, Cretaceous and Cenozoic. The Precambrian–Cambrian period gave rise to the Ara Salt carbonate stringer plays, southern Oman and the giant Zagros anticline traps (Hormuz Formation). While outcrops of salt diapirs are common, basal detachment exposures are extremely rare. The Lakar Kuh area of Central Iran reveals the Precambrian–Cambrian basal salt detachment, on satellite images, in natural cross-section view through the entire Phanerozoic sedimentary section east of Ravar. This view illustrates how older diapiric structures (pillows and normal faults) were the focus of later contractional folds, particularly in the Jurassic and Cenozoic. The salt detachment zone contains many floating blocks (stringers) of clastic, carbonate and igneous rocks. Some blocks were stoped from the overlying beds, while most were layers originally interbedded with the evaporites. Block size, distribution and orientation is highly variable, and folding is infrequent. Lakar Kuh encompasses several key themes generally pertinent to structural geology and salt system research: the presence of sedimentary stringers within evaporites, multiple detachment levels within a thick (>5 km) stratigraphic section, detachment folding, multiphase salt activity, reactivation of older structures by newer ones and multi-stage development of salt bodies.

Some lithogeochemical and isotope-geochemical features of clay rocks and hiatuses in the Riphean stratotype

Research subject. Clay rocks of the Lower, Middle, and Upper Riphean (respectively, the Burzyan, Yurmatau, and Karatau groups) of the Western slope of the Southern Urals. Changes in their characteristic values of a number of ratios of oxides and some trace elements that are indicators of rock composition in provenances are considered, taking into account the breaks of different duration present in the Riphean stratotype. Methods. Calculation of t he average values of TiO2/Al2O3, Th/Sc, La/Co and (La/Yb)N, typical for shales, fine-grained clay siltstones, and mudstones of the main lithostratigraphic units of the Riphean stratotype (formation and, in some cases, subformation) and analysis of their changes taking into account the hiatuses known in the Riphean stratotype (Pre-Mashak/Pre-Zigalga, Pre- Zilmerdak, Pre-Minyar, Pred-Uk). Results. It was established that throughout the entire Riphean, regardless of breaks in sedimentation, presumably associated with certain transformations in the areas of erosion and sedimentation, a number of lithogeochemical characteristics of fine-grained clastic rocks of the Burzyan, Yurmatau, and Karatau groups (such as TiO2/Al2O3, Th/Sc, La/Co, and, to a significant extent, (La/Yb)N) practically did not experience significant changes. Conclusions. The data obtained give reason to think that there were no fundamental changes in the composition of the sources of fine-grained aluminosiliciclastics for the Riphean sedimentary sequences for more than 1 Ga, and contradict the rather active scenario of tectonic processes at the end of the Early Precambrian and the entire Late Precambrian. Conversely, the data on the values of tNd(DM) and eNd(t) in 37 samples of clay rocks of the Riphean stratotype nevertheless reflect a change in the composition of the rocks of the feeding provinces and, one way or another, fit into the outline of subglobal events established by conventional geological methods in the stratotype area of the Riphean. In accordance with them, in the Ai and Mashak times, an increase in the proportion of juvenile material took place in the areas of erosion. Apparently, the Pre-Zilmerdak hiatus also led to the appearance of a significant proportion of basic igneous rocks among the rocks of the provenances.

First Results of the Intraformational Conglomerate Test of the Late Riphean Katav Formation (Southern Urals)

Detailed petrographic, electron-microscopic, and paleomagnetic studies were conducted on flatbedded synsedimentary carbonate breccias from three blocks of rocks (one block from Minyar, two blocks from Katav-Ivanovsk) to verify the ideas about the time of formation of the high-temperature component (HTC) of natural remanent magnetization (Jn) of limestones from the Upper Riphean Katav Formation, Southern Urals. Petrographic and electron-microscopic observations revealed that the composition of pebbles corresponds to that of the host matrix rocks, thus enabling the use of an intraformational conglomerate test. In all the studied blocks, the distribution of paleomagnetic directions for pebbles is generally chaotic, with the concentration parameter not exceeding 3. These test results meet the Graham criterion. The pebbles’ average Jn have a larger confidence oval and differ significantly from the matrix’s directions with a much smaller confidence oval. The Hodges-Ajne test was applied to the blocks from Katav-Ivanovsk. The Rayleigh test was used for the block from Minyar. The results obtained at this stage indicate that the HTC of magnetization of the Katav limestones may have a primary origin. If this is confirmed, the Katav Formation would be a good paleomagnetic record of the Earth’s Neoproterozoic history and provide valuable insights into the geomagnetic field behavior in the Late Precambrian.

Settings for carbon parageneses formation in Late Precambrian

Settings for carbon parageneses formation in Late Precambrian

Basin filling and magmatic response to the migration of two-stage rifts: New insights into the late Neoproterozoic tectonics of the northern Tarim Craton

The spatiotemporal evolution of late Neoproterozoic sedimentary basins and their tectono-magmatic context provide essential information regarding the continental rift process of the Tarim Craton during the fragmentation of the Rodinia supercontinent. However, the incomplete exposure of strata and magmatism in the distal continental margin hinders the understanding of different rift phases that trigger continental breakup. The Sawapuqi region is currently located at the northernmost margin of the Tarim Craton and presents an uninterrupted sedimentary sequence and volcanic rocks from the latest Precambrian to the Cambrian period. Herein, we perform an integrated analysis of the Sawapuqi sub-basin and provide critical insights into the Ediacaran stratigraphic and tectono-magmatic evolution that shed light on the final rifting phase. The Sawapuqi sub-basin preserves regressive and transgressive successions, ranging from thick-bedded siltstones and siliceous rocks at the lower unit of the Sugetbrak Formation, through shore gravel-bearing laminated sandstone accumulation (upper unit), and restricted platform stromatolitic carbonates (Qigebrak Formation), to Cambrian mature passive margin sedimentation. An alkaline basaltic layer is interbedded within the coarsening-upward sedimentary packages of the early Ediacaran, recording the final rift-related magmatic pulse in the northwestern Tarim margin. The geochemical signatures of these basalts suggest a relatively shallow-depth derivation from the enriched mantle reservoir in an intraplate setting. The detrital zircon provenance of the coarse-grained siliciclastic to pebbly feldspathic debris sandstone sequence of the upper Sugetbrak Formation indicates the hybrid sediment infills from local metamorphic basements and Neoproterozoic igneous rocks, with progressively increased contribution from the coeval granites in the northern periphery of the Tarim Craton. The stratigraphic evolution from syn-rift clastic rocks to post-rift carbonates and the transition of detrital provenance observed in the Sawapuqi sub-basin suggest the development of a breakup succession at the end of the early Ediacaran period. The margin-localized continental extension and mantle-derived volcanism during the early Ediacaran differed from the distributed extension and bimodal magmatism during the Cryogenian, indicating two-stage extensional modes and subsidences prior to the lithospheric breakup. The rift system of the latter stage, which migrated oceanwards, eventually led to the continental breakup in the northwestern margin of the Tarim Craton. Comparable late Neoproterozoic stratigraphy and magmatic pulses between the Tarim Craton and Yili-Central Tianshan Block (YCTB) support their reconstructions as two conjugate rifted margins. This study indicates that the rift-drift tectonics triggered by this prolonged episodic rifting between the Tarim Craton and the YCTB was closely associated with the final fragmentation of Rodinia.

ON PALEOTECTONIC BELONGING OF THE SUVANYAK METAMORPHIC COMPLEX (SOUTHERN URALS) FROM THE U-Th-Pb DATING OF DETRITAL ZIRCON GRAINS

U-Th-Pb isotope dating of grains of detrital zircon from quartzites of the Suvanyak metamorphic complex, which forms the Suvanyak tectonic unit that forms the western part of the Uraltau uplift, located in the east of the West Ural megazone in the Southern Urals. The results of isotope dating of grains of detrital zircon from quartzites of the southern part of the Suvanyak metamorphic complex (samples G18-1 and R14-396) show that numerous populations of Late Neoproterozoic–Early Cambrian detrital zircon grains suggest a Peri-Gondwanan origin of the primary sources of detrital material for the protolith of the studied rocks.The structure of the Suvanyak tectonic unit involves metamorphic formations of different ages – Early Paleozoic in the south and Late Precambrian in the north, which are now formally united into a single Suvanyak metamorphic complex. Their differentiation requires additional research.For the Late Paleozoic southeastern margin of the Baltica (at that time already involved in the structure of the composite continent Arct-Laurussia), according to the results of isotope-geochronological study of detrital zircon from the sedimentary and metasedimentary sequences of the Southern Urals, a number of the following tectonic structures were identified. Near the southeastern edge of the margin, there was a Late Neoproterozoic–Early Cambrian oceanic basin, within which a volcanic arc or arcs were active during 650–520 Ma. The structure of the southeastern edge of the margin included the Peri-Gondwanan terrane or terranes (? Cadomian type), as well as thick Riphean-Early Paleozoic sedimentary sequences, autochthonous to the Baltica.

TWO STAGES OF ASSEMBLY OF THE PANGEA SUPERCONTINENT IN THE POLAR URALS: FIRST U/Pb (LA-ICP-MS) AND <sup>40</sup>Ar/<sup>39</sup>Ar AGES OF THE YARKEU COMPLEX

In the Polar Urals, there are few small granitoid intrusions (Yarkeu, Yayu and Pogurei complexes), which are usually associated with the Ural collision. Their Carboniferous-Early Permian age, in most cases, is based on methodologically outdated K/Ar dates, and is also assumed from field geological relationships. We have studied the monzonitoids of the Yarkeu petrotypical pluton, one of the largest such intrusions, and for the first time obtained Late Precambrian U–Pb LA-ICP-MS (zircon) and 40Ar/39Ar (amphibole) ages of 687 ± 3, 669 ± 8 Ma years, respectively. Water-saturated fluid regime (presence of magmatic amphibole and high water content in it, 4.5–5.6 wt %,), hypabyssal formation conditions (P = 2.1–3.5 kbar, T = 871–960°C), features of the chemical composition of monzonitoids (low TiO2 (0.6–0.8 wt %) and CaO (3.6–6.3 wt %) contents, high Al2O3 (16.6–18.0 wt %) and K2O (1.7–4.6 wt %), enrichment in large-ion lithophile elements relatively high field strength and rare earth elements, Ta-Nb minimum and Pb maximum), as well as metallogenic specialization in Cu and Mo (±Pb, Hg, Au), indicate the formation of the rocks of the Yarkeu complex in a subuction-related setting in a fairly thick continental crust. Isotopic clocks in these monzonitoids captured two stages of the assembly of the Pangea supercontinent: the earliest stage in the Late Precambrian, when monzonitoids were formed on the active Bolshezemel margin of Arctida, and one of the final stages in the Middle to Late Carboniferous, when the rocks underwent metamorphism due to the closure of the Paleo-Ural ocean and the onset of collision between Arkt-Laurussia and Siberia.

Extreme variability in atmospheric oxygen levels in the late Precambrian.

Mapping the history of atmospheric O2 during the late Precambrian is vital for evaluating potential links to animal evolution. Ancient O2 levels are often inferred from geochemical analyses of marine sediments, leading to the assumption that the Earth experienced a stepwise increase in atmospheric O2 during the Neoproterozoic. However, the nature of this hypothesized oxygenation event remains unknown, with suggestions of a more dynamic O2 history in the oceans and major uncertainty over any direct connection between the marine realm and atmospheric O2. Here, we present a continuous quantitative reconstruction of atmospheric O2 over the past 1.5 billion years using an isotope mass balance approach that combines bulk geochemistry and tectonic recycling rate calculations. We predict that atmospheric O2 levels during the Neoproterozoic oscillated between ~1 and ~50% of the present atmospheric level. We conclude that there was no simple unidirectional rise in atmospheric O2 during the Neoproterozoic, and the first animals evolved against a backdrop of extreme O2 variability.

DICKINSONIA COSTATA of the Winter Mountains: Features of morphology and ontogenesis

The remains of Ediacaran (Vendian) macrofossils from the Zimnie Gory (Winter Mountains) locality (Russia, southeastern White Sea area) are used to characterize one of the classic animals of the Late Precambrian, Dickinsonia costata. The Zimnie Gory material shows that the body of this species, like that of other species of Dickinsonia, was divided into “half-segments” (isomers) arranged in two rows in alternating order. This makes it possible to classify Dickinsonia as an Ediacaran phylum of Metazoa, Proarticulata. D. costata appeared in the benthic community with a body diameter of about 1 mm and the possible absence of transverse differentiation. D. costata from Zimnie Gory grew to very large sizes, reaching a length of 390 mm and having at least 225 pairs of isomers. During the ontogeny of D. costata, there was a significant change in the shape and relative size of the anterior lobe and a multiple increase in the number of isomers. At the same time, in contrast to other dickinsonias, the general body proportions of D. costata from Zimnie Gory varied slightly towards a slight decrease in the relative width. The addition of new isomers occurred during the entire observed interval of the life cycle of this Dickinsonia. Based on this, the growth pattern of D. costata is defined as allometric and indeterminate.

VENDIAN IN EXPLANATORY NOTES TO SHEETS OF STATE GEOLKARTA-1000/3 N-40 – Ufa and O-40 – Perm: QUESTIONS OF AN AMATEUR

The article critically analyzes the information about the Vendian summarized in the Legend of the Ural series of sheets of Gosgeolkarta-1000/3 and explanatory notes to its sheets N-40 – Ufa and O-40 – Perm. It is concluded that the authors of these materials failed to carry out a scientific generalization and interpretation of all previously obtained geological, geophysical, geochemical and other materials and, on this basis, create a general model of the Vendian stage of development of the territory of the Southern and Middle Urals at the end of the Late Precambrian.

A Devious Equatorial Dipole Hypothesis: on the Low-Latitude Glaciations Problem and Geomagnetic Field Configuration in Late Precambrian

Abstract—The analysis of paleomagnetic data from Late Neoproterozoic complexes of Siberia and Australia is carried out. We show that the existing paleomagnetic datasets are in a disagreement with the concept of the axial-dipole configuration of the Late Neoproterozoic geomagnetic field: proposed non-actualistic models of the field do not reasonably explain the distribution of the paleomagnetic poles. We carried out analysis of paleomagnetic and virtual geomagnetic poles distribution based on simple geometric calculations. The analysis suggests that the configuration of the Late Neoproterozoic geomagnetic field was determined by the coexistence of a weak, long-lived source that was stably fixed in space, with a main dipole source that experienced sporadic multidirectional jumps within a certain preferred region of the Earth. Predominantly equatorial orientation of the main dipole source is substantiated by paleoclimate proxies. We propose a descriptive non-actualistic model of the Late Neoproterozoic geomagnetic field—the Devious Equatorial Dipole hypothesis, which brings paleomagnetic and paleoclimate data into accordance.

Geochemical Proxies of Endo- and Exospheric Processes in Sedimentary Sequences and the Upper Precambrian Section in the South Urals

The article characterizes some geochemical proxies of endo- and exospheric processes in sedimentary sequences and analyzes their average values in Upper Precambrian clayey rocks in the South Urals. It is shown that a gradual decrease of the Th/Scaver value in Lower Riphean (Mesoproterozoic) ~1750–1300 Ma ago rocks correlates to a certain extent with the assembly and disintegration of the Columbia/Nuna supercontinent. The assembly of Rodinia corresponds to two peaks (Biryan Member and Min’yar Formation) of relatively high Th/Scaver values. However, they are separated by rocks with Th/Scaver values comparable with those typical of the fine-grained clastic rocks of the Mashak plume episode (~1380 Ma). Variations of Th/Craver in the Lower–Middle Riphean clayey rocks also correlate quite well with the assembly of Columbia/Nuna, whereas the Upper Riphean–Vendian (Neoproterozoic) interval does not show any significant variation in Th/Craver values. Hence, the assembly and disintegration of Rodinia and the associated changes in rock composition in the provenance did not have any notable effect on the fine-grained aluminosilicate rocks of the Riphean stratotype. The Middle–Upper Riphean sequences were also formed at relatively constant values of TDM and eNd(T). Hence, the Grenvillian events did not make significant contribution to the formation of rocks of the Yurmatau and Karatau groups. Average values of indicators of the redox parameters of water in the Late Precambrian basins, which existed in the eastern part (in modern coordinates) of the East European Platform, indicate that their sedimentary infilling was deposited predominantly under oxidizing or similar conditions. This conclusion contrasts markedly with the notion of the prevalence of ferruginous and euxinic environments in the World Ocean during the Late Precambrian. Comparison of variations in the mean EFNi and EFZn values in clayey rocks of the South Urals, as well as other paleoproductivity proxies, does not reveal any definite relationship of their excursions with the Late Precambrian global and subglobal, endo- and exospheric events. In the Early–Middle Riphean succession, the P2O5 values, slightly higher than in PAAS, correspond approximately to the period of Columbia/Nuna supercontinent existence. Lower values are typical for the clayey rocks formed during its disintegration, although the volume of mafic igneous rocks (main sources of phosphorus) in the provenances at that time should be larger. During the existence of Rodinia, the Upper Riphean clayey rocks were characterized by P2O5aver values both higher and lower than the PAAS. The same is true for its disintegration period. These facts suggest that the global and subglobal, endo- and exospheric processes did not significantly affect the formation of the Riphean–Vendian sedimentary sequences in the South Urals. More important here is the role of local factors.

Conical Thecae of Precambrian Macroorganisms

On the basis of new extensive collections, made by the present authors, a group of unsegmented three-dimensional fossil remains from the Late Precambrian of Namibia and the southeastern White Sea area, including such genera as Protechiurus, Vendoglossa, and Vendoconularia, were restudied. It is established that the fossils belong to two morphologically close genera (Protechiurus and Vendoconularia), composing the new family Protechiuridae. It is possible that they represent the same genus or even species, but the incompleteness of the preservation of the Namibian member of the group prevents their integration. The fossils initially represented conical hollow elastic objects, hexagonal in the cross-section, which were open at the wide end and closed at the pointed end. The sculpture of their surface consisted of longitudinal ridges, both single and double, bounding the rows of the short transverse ridges, scalloped along one margin. Vendoconularia had wide plates which were attached to the cone outwardly along the longitudinal unpaired ridges. In general morphology and several details the fossils resemble the thecae of conulariids and anabaritids, known from Late Precambrian and Paleozoic and classified as the scyphozoan cnidarians. The main difference lays in the theca mineralization, which consists of phosphate in conulariids, carbonate in anabaritids, and is absent in the case of protechiurids. It is proposed that the protechiurids, which appeared in the fossil record first, may belong to the basal group of ancient scyphozoans, ancestral to the conulariids on the one hand, and to the anabaritids on the other.

Delineation of radiometric anomalies and conductive zones using gamma-ray spectrometric and electromagnetic methods, east Gabal El-Urf area, Northern Eastern Desert, Egypt

Ground gamma-ray spectrometric (GRS) and horizontal-loop electromagnetic (HLEM) surveys were carried out to determine the radioactive anomalous zones and to follow their lateral and vertical extensions in east Gabal (G.) El-Urf area, Northern Eastern Desert, Egypt. This area and its surroundings are mainly covered by basement rocks of Late Precambrian age, represented by quartz diorites and granite porphyry, which are intruded by pegmatites, quartz veins, and basic dykes. These rocks are traversed by dry wadis (dry valleys), which are filled by Quaternary sediments.The ground gamma-ray spectrometric data revealed that the study area possesses a wide range of radioelement concentrations, oscillating from 37 to 2164 μR/h, 0.9–170 ppm, 0.1–312 ppm, and 0.2%–7.8% for total count (TC), equivalent uranium (eU), equivalent thorium (eTh), and potassium (K), respectively. The eU composite image showed that high radioactive parts are mainly associated with the pegmatite rocks.Ground HLEM survey was performed at four frequencies (7040 Hz, 1760 Hz, 440 Hz, and 110 Hz) in the study area. The interpretation of the HLEM data indicated the presence of various conductive zones, mainly associated with pegmatite bodies. The parameters of source causative bodies, such as location, dip angle, width, and conductivity thickness were estimated for three conductive bodies. The calculated depths to the selected HLEM anomalies appear to be shallow, varying from 10 m to 22 m. All the anomalous bodies have dips ranging between 60° and 80° toward the south and west. The widths of the anomalous bodies range from medium conductors (13 m) to broad conductors (40 m or more). The conductivity thicknesses showed a wide range (1.14–72.7 Siemens), with the highest value associated with the highest eU anomaly.

Age and Depositional Settings of the Ordovician Chalovskaya Group in the Argun Massif, Eastern Part of the Central Asian Fold Belt

The results of mineralogical and geochemical studies of the metaclastic rocks of the Grebnev and Magdagachi sequences of the Chalovskaya Group, one of the key lithostratigraphic units of the Argun continental massif in the eastern part of the Central Asian Fold Belt, as well as the results of the U–Pb dating of detrital zircons from these rocks by the LA-ICP-MS method, are presented. It was found that the age of the youngest zircons is ~478 Ma (Early Ordovician) in the metasandstones of the Grebnev Sequence and ~448 Ma (Late Ordovician), in the metasiltstones of the Magdagachi Sequence. Together with the previously determined age of the metadacites of the Isagachi Sequence (476 ± 8 Ma), this suggests the Ordovician age of the Chalovskaya Group, dated previously as Late Precambrian. The available data indicate that the deposits of the Chalovskaya Group accumulated in a setting of an active continental margin or an island arc against the background of intense magmatic activity. Considering that the Early Paleozoic suprasubduction magmatism is manifested in all continental massifs of the eastern part of the Central Asian Fold Belt (Argun, Mamyn, Bureya, Jiamusi, Khanka), one cannot exclude that these continental massifs have a common geodynamic history.

Age and formation features of the Ordovician Chalovskaya sequence in the Argun Massif, eastern part of the Central Asian fold belt

In this paper we present the results of the mineralogical and geochemical studies of metaterrigenous rocks of the Grebnev and Magdagachi formations of the Chalovskaya Sequence – one of the key stratigraphic divisions of the Argun continental Massif in the eastern part of the Central Asian fold belt, and U–Pb geochronological (LA-ICP-MS) data for the detrital zircons from these rocks. It is established that the youngest zircons in the metasandstones of the Grebnev Formation have an age of ~478 Ma, and in the metaalevrolites of the Magdagachi Formation ~448 Ma. Together with the previously obtained data on the age of metadacites from the Isagachi Formation (476 ± 8 Ma), this information indicates the Ordovician age of the Chalovskaya Sequence, previously attributed to the Late Precambrian. It was shown that the accumulation of sediments of the Chalovskaya Sequence took place in an environment of an active continental margin or island arc simultaneously with intense magmatic activity. Taking into account the fact that the Early Paleozoic suprasubduction magmatism manifested within all the continental massifs of the eastern part of the Central Asian fold belt (Argun, Mamyn, Bureya, Jiamusi, Khanka), it cannot be excluded that the mentioned continental massifs have a common geodynamic history.

Structure and development of the Valmiera–Lokno Uplift – a highly elevated basement block with a strongly deformed and eroded platform cover in the East European Craton interior around the Estonian–Latvian–Russian borderland

Based on drillings, a number of geological cross-sections, and structure contour and isopach maps were composed to describe/analyse the structure and development of the Valmiera–Lokno Uplift (VLU), a basement block elevated up to 700 m with a heavily deformed and eroded platform cover in the East European Craton interior, along the regional Liepaja–Riga–Pskov Fault Zone (LRPFZ). Five isolated basement-cored anticlines (BCA), the Lokno, Haanja, Moniste, Valmiera and Smiltene uplifts, arise in the platform cover on the VLU, whereas the downthrown LRPFZ side defines a complex monoclinal fold. The anticlines, straddling or occurring near the monocline, merge with it and thus have highly asymmetrical shapes. Thickness changes of stratigraphic units across the VLU reveal its complex history, reflecting regional tectonic activation pulses that varied noticeably even between neighbouring BCAs. In all, the latest Precambrian–earliest Ordovician initiation epoch of the VLU was followed by modest tectonic activity or a standstill period in the Middle Ordovician–Early Silurian. Intensifying tectonic movements culminated again in the prime of the Caledonian Orogeny in latest Silurian–earliest Devonian time, and faded thereafter towards the end of Early Devonian. The VLU has been reactivated occasionally since the latest Devonian and emerges as a crustal weakness in the recent movement and seismicity patterns. To decipher the origin of the VLU, hitherto factually undiscussed topics, a more detailed study of the LRPFZ, analysis of its fault pattern and kinematics alongside the regional tectonic setting/history is needed. A cursory look hints to a substantial Early Paleozoic sinistral strike-slip along the LRPFZ, allowing interpreting the VLU as a possible restraining bend structure.

Evolution of the Karebas Fault System and adjacent folds, central Zagros fold-and-thrust belt, Iran: Role of pre-existing halokinesis (salt structures and minibasins) and detachment levels

In the central part of the Zagros fold-and-thrust belt of Iran, several diapirs of the Latest Precambrian–Early Cambrian Hormuz Salt locally decorate the surface exposures of a series of N–S to NNW–SSE trending right-lateral tear fault systems. The Karebas Fault System (KBFS), as one of these fault systems, is associated with five salt diapirs. There are also several NW–SE trending anticlines, which are either cross-cut by the fault system or terminated against it and the adjacent salt diapirs. The Neogene Zagros folds are clearly affected by the surface expression of the Karebas Fault System, suggesting that the fault system is coeval with or post-dates the Neogene deformation. On the contrary, halokinesis as old as at least Late Jurassic is documented by near-diapir stratigraphic exposures, indicating pre-orogenic existence of the diapirs. Although the lack of older rock outcrops precludes tracking the pre-Jurassic records of diapirism, the geometry of strata, which flank these diapirs, indicates long-term pre-folding salt rise driven by downbuilding of lateral minibasins and progressive draping of sedimentary layers over the flanks of the rising diapirs. Therefore, presence of old salt structures along the young (Neogene) tear fault system suggests an important role played by the presence of early-formed salt structures in inducing later strike-slip deformation along zones of preferential weakness, which was prepared by the pre-folding salt structures and thinner sedimentary pile overlying them. Salt diapirs and related changes in stratigraphic thicknesses (i.e. minibasins) also effectively grappled with the process of folding. Detachment folding is typically the main structural style across the area, and was mainly developed above the Hormuz Salt as the basal décollement and partly by involvement of the intermediate detachment levels. However, preferential weakness and squeezing of the diapirs on one hand and greater strength of the minibasins on the other hand gave rise to further complexities of deformation during shortening. Such first-order controls could led to perturbations, at the scale of diapirs and minibasins, in the stress pattern as the southwestward-advancing front of the Zagros deformation migrated across the stratigraphic section and the salt structures. It occurred because strain is localized at the diapirs and squeezes them whereas tends to avoid folding the thick, and hence stronger, minibasins into anticlines. In such a context, the localization and the longitudinal propagation of the folds in the cover sequence could be remarkably influenced and irregular. The consequence was variable fold geometries (location and dimension) and fold-axes rotations with respect to the diapirs and minibasins.