The geodynamic evolution of the folded framing and the western margin of the Siberian craton in the Neoproterozoic: geological,structural, sedimentological,geochronological, and paleomagnetic data

The formation of the western margin of the Siberian craton in the Neoproterozoic is considered, with a focus on its transformation from a passive continental margin into an active one, accretion and collision processes, formation of island arcs and ophiolites, orogeny, and continent-marginal rifting. The evolution and correlation of sedimentary basins within fold-thrust belts of the Siberian Platform framing are considered. New structural and kinematic data on the Yenisei fault zone are discussed. On the basis of paleomagnetic data obtained for the structures in the zone of junction of the Siberian Platform and the West Siberian Plate, new models are proposed for the location of the Siberian craton relative to other paleocontinents and microcontinents in the Neoproterzoic. All these data provide a consistent evolution scheme for the western margin of the Siberian paleocontinent in the Neoproterozoic and constrain the position of the Siberian craton margin in Late Neoproterozoic (pre-Vendian) time.     

The mesozoic evolution of the mino terrane, central Japan: A geologic and paleomagnetic synthesis

The Mino tectono-stratigraphic terrane, central Japan, underlain by Permian to Jurassic sedimentary and volcanic rocks of various origins, was formed through accretion processes associated with the Mesozoic sea-floor spreading. This conclusion has been reached mainly from the following reasoning:

1. (1) the entire boundary of this terrane is defined by tectonic belts with high-pressure metamorphic rocks and serpentinized ultramafic rocks,

2. (2) the chemistry and petrology of the Permian greenstones demonstrate their affinity with abyssal tholeiitic and alkalic basalts,

3. (3) the widespread, but chaotic, occurrence of Permian greenstones, Triassic cherts, and Jurassic siliceous shales in the younger Jurassic clastic rocks of this terrane suggests extensive post-depositional mixing of strata,

4. (4) the sedimentology of the Jurassic sandstones strongly suggests that they are turbidity-current deposits supplied from cratonic lands,

5. (5) the South-Pacific type fossil assemblage in the Mino terrane shows strong contrast with the North-Pacific type fossil assemblage of the adjacent terranes,

6. (6) the paleomagnetism of the Permian and Jurassic greenstones, the Triassic cherts, and the Jurassic siliceous shales implies long-distance northward drift in Cretaceous time of these rocks from their original low latitudinal regions.

Along with this northward migration, the Mino terrane was accreted with extensive internal deformation to northeast Asia including the present Hida terrane. Recent accumulation of paleomagnetic and paleontologic data in the Pacific peripheral regions appears to support the existence of many allochthonous terranes which migrated from the equatorial regions. The Mino terane may be regarded as one example of these Circum-Pacific allochthons.     

The Karelian Craton in the Paleoproterozoic: New paleomagnetic data

New paleomagnetic data on Paleoproterozoic complexes of the Central Karelian and Vodlozero terranes of the Karelian Craton were obtained. A new key paleomagnetic pole (1.98 Ga) was calculated for rocks of the Vodlozero terrane. The positions of Central Karelian and Vodlozero terranes 1.98 Ga ago in subtropical and moderate latitudes of the Northern Hemisphere, respectively, were reconstructed. The latitudinal difference (1.98 Ga) between the positions of Central Karelian and Vodlozero terranes supports the existence of oceanic basins between separate terranes of the Karelian Craton.     

Neoproterozoic evolution of Rodinia: constraints from new paleomagnetic data on the western margin of the Siberian craton

The paper summarizes paleomagnetic results obtained from the Neoproterozoic rocks of the western margin of the Siberian craton. On the basis of the obtained paleomagnetic poles and available paleomagnetic data for the Precambrian of Siberia, a new version of the Neoproterozoic segment of the apparent polar wandering path (APWP) is proposed for the craton and is compared with the Laurentian APWP. The superposition of these paths suggests that in the Neoproterozoic the southern margin of the Siberian craton (in modern coordinates) faced the Canadian margin of Laurentia. Most likely, in the end of the Mesoproterozoic and during the Neoproterozoic the Siberian craton and Laurentia were connected to form the supercontinent Rodinia. At 1 Ga the western margin of the Siberian craton was a northern (in modern coordinates) continuation of the western margin of Laurentia. The available paleomagnetic data on Laurentia and continental blocks of Eastern Gondwana (Australia, Antarctica, India, South China) and the proposed APWP trend allowed a new model for the breakup of this segment of Rodinia. Analysis of a total of the data available suggests that strike-slip movements on the background of the progressive opening of the oceanic basin between Siberia and Laurentia were predominant in the south of the Siberian craton during the Neoproterozoic. Similar kinematics is typical of the western margin of Laurentia, where strike-slip motions are probably associated with the progressive opening of the ocean basin between Laurentia and eastern Gondwana.     

The East European Platform in the late Ediacaran: new paleomagnetic and geochronological data

The paleogeography of the Earth, including the East European Platform, is very inaccurately defined for the interval 500–700 Ma. The quantity and quality of Late Precambrian–Cambrian paleomagnetic data on this platform are absolutely insufficient for reliable paleogeographical or paleotectonic reconstructions. Since there are almost no unstudied objects in the platform that could be used for paleomagnetic studies, it seems reasonable to consider the deformed platform margins. Of particular interest is the Bashkir anticlinorium (South Urals) with numerous Ediacaran sedimentary sections, some of which contain tuff beds suitable for isotope dating. We present paleomagnetic and geochronological data on the Upper Ediacaran Zigan Formation, sampled in the western part of the western limb of the Bashkir anticlinorium. The East European Platform must have been at near-equatorial latitudes at ～550 Ma.     

Depositional history and stratigraphical evolution of the Sakoa Group (Lower Karoo Supergroup) in the southern Morondava Basin, Madagascar

The Sakoa Group is the lowermost stratigraphical succession of the Karoo Supergroup and the oldest sedimentary unit in Madagascar, spanning the Late Carboniferous through Early Permian epochs. The Sakoa Group is exposed in the southern Morondava Basin. It is predominantly a siliciclastic sequence comprising seven lithofacies associations: (1) diamictites; (2) conglomeratic sandstones; (3) sandstones; (4) interbedded thin sandstones and mudstones; (5) mudstones; (6) coals; and (7) limestones. These facies represent deposition in the early extensional stages of continental rift development. The sediments were deposited predominantly on alluvial fans, and in braided to meandering stream and overbank environments. Locally lacustrine and coal swamp environments formed in low areas of the basin floor during rift initiation. Subsidence rates remained fairly constant throughout the Early Permian and were accompanied by a gradual reduction in relief of the basin margins and an increased geomorphic maturity of the fluvial systems flowing across the basin floor. Near the end of the Early Permian the southern Morondava Basin was inundated by a marine transgression , which resulted in deposition of the Vohitolia Limestone. Subsequent tectonic uplift and erosion resulted in a regional unconformity between the Sakoa Group and the overlying Sakamena Group.     

Paleogeographic evidence on the Jurassic tectonic history of the Pontides: new paleomagnetic data from the Sakarya continent and Eastern Pontides

The Jurassic paleogeographic position of the Pontides is not well studied because of insufficient paleomagnetic data. For this reason, a paleomagnetic study was carried out in order to constrain the paleolatitudinal drift of the Turkish blocks during the Jurassic period. A total of 32 sites were sampled from volcanic and volcanoclastic rocks of the Lower/Middle Jurassic Kelkit formation (Eastern Pontides), Mudurnu formation (Sakarya continent) and Upper Jurassic–Lower Cretaceous Ferhatkaya formation exposed around Amasya region (Eastern Pontides). Rock magnetic experiments demonstrate that the main ferromagnetic mineral is pseudo-single-domain titanomagnetite in these rocks. Paleomagnetic analysis revealed two main components of the natural remanent magnetization during stepwise thermal and alternating field demagnetization. The first component is a low-coercivity (unblocking temperature) component with a direction sometimes similar to that of the earth’s present field or a viscous component. The second component, which is interpreted as the characteristic remanent magnetization (ChRM) direction, has low to high coercivity properties between 20 and 100 mT or unblocking temperatures between 300 and 580°C. A positive fold test at the 95% level of confidence proved that the ChRM of the sites is primary. Paleomagnetic directions calculated for the Kelkit formation in the Eastern Pontides have a mean direction of D = 334.8°, I = 49.7°, α ₉₅ = 7.1° after tilt-correction. A mean direction of D = 332.2°, I = 48.5°, α ₉₅ = 14.6° was obtained from the volcanoclastic rocks of the Mudurnu formation, and D = 324.3°, I = 43.3°, α ₉₅ = 9.5° was calculated for the Upper Jurassic–Lower Cretaceous limestones/Ferhatkaya formation of the Amasya region. The Jurassic rocks in the Eastern Pontides and Mudurnu region are considered to represent products of the rifted Neo-Tethys ocean, while the Upper Jurassic–Lower Cretaceous sediments in Amasya are related to basin-filling materials. The data suggest that the Kelkit formation was formed at 30.5°N paleolatitude and the equivalent Mudurnu formation at 29.5°N paleolatitude. The paleolatitude of the Eastern Pontides indicates that this rifting block was separated from Eurasia by a marginal basin instead of being a part of Eurasia. The lower paleolatitude of the Amasya region at 24.8°N in the Upper Jurassic to Lower Cretaceous clearly indicates southward drift of the Turkish blocks during the Jurassic to Lower Cretaceous period together with the motion of Eurasia.     

Depositional history and stratigraphical evolution of the Sakamena group (Middle Karoo Supergroup) in the southern Morondava Basin, Madagascar

The Karoo Supergroup in Madagascar is subdivided into three lithostratigraphical units: the Late Carboniferous-Early Permian Sakoa Group; the Late Permian-Middle Triassic Sakamena Group; and the Triassic-Early Jurassic Isalo Group. The Sakamena Group is fairly well exposed in the southern Morondava Basin, where it is approximately 4000 m thick. The Sakamena Group is separated from the Sakoa Group by an angular unconformity. The Lower Sakamena Formation is characterised by two major facies associations: (1) interbedded muddy conglomerates and coarse sandstones; and (2) interbedded sandstones and mudstones, which were deposited in a rejuvenated rift setting by coarse-grained fluvial systems and debris flows on the rift margins. In the Vatambe area, facies represent fandelta deposition in a saline lake or tongue of the ocean. The Middle Sakamena Formation comprises three major facies: (1) laminated mudstones and sandstones; (2) sandstones; and (3) mudstones. The Middle Sakamena facies were deposited by low gradient meandering streams and in shallow lakes. The Upper Sakamena Formation was deposited in similar environments, except that it is comprised predominantly of red beds. The Isalo Group consists predominantly of coarse-grained sandstones (up to 6000 m thick). These sandstones were deposited by braided streams with the coarse detritus derived from a structural uplift in the east.     

Late Eocene and Maastrichtian paleomagnetic poles for the Pacific plate: implications for the validity of seamount paleomagnetic data

Mean paleomagnetic poles for the Pacific plate have been calculated for the Late Eocene (39 Ma) and the Maastrichtian (69 Ma). The former is located at 77.6°N, 7.6°E, the latter at 69.9°N, 0.9°E. Although these pole positions are little changed from previous calculations they are better constrained with additional data. Slightly less than half of the data are derived from the inversion of seamount magnetic fields providing an excellent opportunity to compare such data with other paleomagnetic data of the same age. As no significant systematic difference between the two types of data is evident, it is inferred that most seamount paleomagnetic data are probably useful indicators of the paleomagnetic field direction.     

Late Triassic initial closure of the Mongol-Okhotsk Ocean in the western segment: Constraints from sedimentological,detrital zircon ages and paleomagnetic evidence

The Mesozoic evolution of the Mongol-Okhotsk Ocean (MOO) has significantly affected the configuration of the modern Asian continent. Although a scissor-like closure of the MOO has long been proposed, when and how the MOO closed are still hotly debated, especially the timing of initial closure of the MOO in its western segment, hindering our understanding of both the evolution of the MOO and tectonics of the northern Asian continent. In order to uncover the timing of initial closure of the MOO, we performed a multidisciplinary study in sedimentology, detrital zircon U-Pb dating and paleomagnetic on the Late Triassic clastic strata from the Tarvagatay Block and the Amuria Block (AMB) on the both sides of the Mongol-Okhotsk Suture. The upper Triassic strata on both sides of the suture were dominated by plant fossil-bearing alluvial-fluvial facies sediments, which unconformably overlain pre-Triassic geological units, indicating a terrestrial setting after the closure of the MOO. Detrital zircon U-Pb dating results revealed consistent age distribution patterns for samples from both sides of the suture with a predominant peak at ∼253–251 Ma and a secondary peak at ∼359–357 Ma, representing two main arc magmatic events during the bidirectional subduction of the MOO in the Late Devonian-Early Carboniferous and Late Permian-Early Triassic. Coeval Late Triassic paleomagnetic poles were obtained from the northern AMB and Tarvagatay Block, revealing a comparable paleolatitude of the AMB (∼31–33°) and Tarvagatay Block (∼32–34°) in the Late Triassic, arguing for that the western segment of the MOO should have closed at the Late Triassic. The compilation of sedimentology, detrital zircon U-Pb dating, magmatic and paleomagnetic evidence provides integrated constraints on the Late Triassic initial closure of the MOO in its western segment.     

Cambrian to Devonian evolution of alluvial systems: The sedimentological impact of the earliest land plants

In present-day alluvial environments, the impact of vegetation on sedimentological processes and deposits is well known. A vegetated catchment may decrease sediment yield, sediment erodibility, Hortonian overland flow, aeolian winnowing of fines, the proportion of sediment transported as bedload, and may increase bank stability, infiltration into substrates, and bed roughness. Vegetation also promotes the production of chemically-weathered clays and soils and the adoption of a meandering style. It is generally understood that, prior to the evolution of terrestrial vegetation during the Early Palaeozoic, ancient alluvial systems were markedly different from modern systems, with many systems adopting a “sheet-braided” style. This understanding has previously informed the interpretations of many Precambrian pre-vegetation alluvial successions, but there has been relatively little work regarding Early Palaeozoic alluvial successions laid down prior to and during the initial colonization of the Earth's surface by plants.A comprehensive review of 144 Cambrian to Devonian alluvial successions documented in published literature was combined with original field data from 34 alluvial successions across Europe and North America. The study was designed to identify changes in alluvial style during the period that vegetation was evolving and first colonizing alluvial environments. An increase in mudrock proportion and sandstone maturity is apparent, along with a decrease in overall sand grain size through the Early Palaeozoic. These trends suggest that primitive vegetation cover promoted the production and preservation of muds from the mid Ordovician onwards and increased the residence time of sand-grade sediment in alluvial systems. The compilation also enables the first stratigraphic occurrence of certain vegetation-dependent sedimentary features to be pinpointed and related to the evolution of specific palaeobotanical adaptations. The first markedly heterolithic alluvial sequences appeared at about the same time as the most primitive terrestrial vegetation in the Ordovician, and prolific pedogenic calcite, charcoal and bioturbated floodplain fines first appeared in the rock record at about the same time as vascular-plant macrofossils became abundant in the late Silurian. Lateral accretion sets in channel deposits appeared near the Silurian–Devonian boundary, at or shortly before the appearance of underground rooting systems, and become progressively more abundant in the record during the Devonian, implying a major expansion of meandering rivers as rooted plants stabilized river banks. Coals become abundant after the development of plant arborescence. The analysis suggests that the evolution of embryophytes had a profound effect on fluvial processes and deposits, and this period of landscape evolution must be considered amongst the most significant environmental and geomorphological changes in Earth history, with profound consequences for all aspects of the Earth system.     

Precambrian microcontinents of the Ural-Mongolian Belt: New paleomagnetic and geochronological data

The knowledge on the early stages of evolution of the Ural-Mongolian Belt (UMB) (Late Neoproterozoic-Cambrian) is a key for understanding of its evolution in the Paleozoic. Unfortunately, this stage remains poorly studied. The tectonic reconstructions of the UMB for this time primarily depend on the views on the kinematics and tectonic evolution of numerous sialic massifs with Precambrian basement in the structure of the Tien Shan, Kazakhstan, Altai, and Mongolia. At present, the concept of the origin of these massifs is largely based on the lithostratigraphic similarity of the Neoproterozoic and Lower Paleozoic sections of the Tarim, South China, and Siberian platforms with coeval sections of Precambrian massifs within the UMB. New paleomagnetic and geochronological data can serve as additional sources of information on the origin and paleotectonic position of the microcontinents. In this paper, we present new isotopic datings and a new paleomagnetic determination for the Neoproterozoic volcanic rocks of the Zabhan Formation from the Baydrag microcontinent in central Mongolia. It is established that 805−770 Ma ago (U-Pb LA-MC-ICP-MS age of zircon) the Baydrag microcontinent was situated at a latitude of 47 ± 14° in the Northern or Southern hemisphere. These data provide new insights into the possible origin of the Precambrian micro-continents in the UMB. Analysis of paleomagnetic data and comparison of the age of the basement beneath various plates allow us to state rather confidently that ∼800 Ma ago the micro-continents of the UMB belonged to one of the North Rodinian plates: Indian, Tarim, or South China; their Australian origin is less probable.     

The sedimentological evolution of a Lower Palaeozoic accretionary fore-arc in the Southern Uplands of Scotland

Thick turbidites accumulated along the northern margin of the Iapetus Ocean in Britain from mid-Ordovician to late Silurian times. Recent plate tectonic reconstructions hold that, during subduction, packets of these sediments, together with the underlying pelagic facies and thin portions of the uppermost ocean crust, were stripped from the descending plate and accreted to the inner trench wall on the Laurentian (North American) continental margin. The resulting accretionary prism is represented today by the Ordovician and Silurian rocks of the Southern Uplands of Scotland and the Longford-Down massif of Ireland. In these areas major reverse faults separate tracts of steeply dipping greywackes and mudstones with minor amounts of cherts and basalts. These tracts are up to several kilometres wide; their constituent beds face predominantly to the northwest, away from the site of the ancient ocean, while becoming progressively younger in each major fault slice towards the Iapetus suture in the southeast. From the stratigraphic sequences in these fault slices the sedimentary history of a portion of the Iapetus Ocean, and the British sector of its northern margin, can be reconstructed. In the Southern Uplands the earliest turbidites (mid- and late-Ordovician) are preserved in the northernmost fault slices. Regional facies trends, and vertical facies analysis, suggest that they accumulated in a trench dominated by a series of relatively small lower trench slope-derived fans. Pelagic sediments of the same age are found in the fault slices to the south, suggesting that the Ordovician turbidites were confined to the trench. During the lower and middle Llandovery, volcaniclastic trench turbidites were separated from quartz-rich ocean-floor turbidites (represented in the southern fault slices) by an elongate rise, on which pelagic deposits accumulated. This is interpreted as the outer trench high. In late Llandovery times the rise was overwhelmed, and thick laterally derived quartzose turbidites blanketed both the trench and the ocean floor. Sedimentation was strongly influenced by the evolution of the accretionary prism. By Llandovery times a trench slope break had emerged, supplying sediment both south to the trench and north to an upper slope basin in the Midland Valley of Scotland. In this basin early Silurian turbidites were followed by shallow-water and terrestrial sediments. Most of the sediment was derived from the emergent trench slope break: the volcanic arc and the Grampian orogenic belt to the north provided little or no detritus. Throughout the Ordovician and Silurian, sediment in the trench and on the ocean floor was derived from the volcanic arc, from the lower trench slope/trench slope break, from a degrading plutonic/metamorphic terrain (the Grampian Orogen), and locally by a minor amount of submarine sliding from carbonate-capped volcanic seamounts. Progressive elevation of the trench slope break in Silurian (and perhaps late Ordovician) times indicates that sediment from the arc-orogen hinterland must have bypassed the upper slope in the unexposed section of the margin to the northeast of the Southern Uplands, and travelled into the area axially along the trench floor.     

The Black Reef Quartzite Formation in the western Transvaal: sedimentological and economic aspects,and significance for basin evolution

A sedimentological study of the early Proterozoic Black Reef Quartzite Formation in the south-western parts of the Transvaal province of South Africa was undertaken with the primary aim of examining the sedimentological controls of gold mineralization in the Black Reef placer, which occurs at the base of this Formation. A second aim of the study was to investigate the early history of the basin in which the Transvaal Sequence of South Africa was deposited. The thin, siliciclastic Black Reef Quartzite Formation, which is informally subdivided into a lower Conglomerate Unit and an upper Quartzite Unit, is underlain by Archaean rocks belonging to the basement complex and the Witwatersrand and Ventersdorp Supergroups, and is overlain by a thick succession of carbonate rocks of the Malmani Subgroup. The pre-Transvaal palaeosurface is characterised by elongated northeast to southwest trending grabens and partly-eroded horst blocks. The Black Reef Quartzite Formation, which has a maximum thickness of about 30 m in the study area, typically comprises a succession of interbedded arenites and mudstones, with a sporadically-developed basal Conglomerate Unit. Thickness trends are similar to the dominant structural trend of the pre-Transvaal palaeosurface. At localities where the Conglomerate Unit is absent, the Formation invariably overlies quartzites of the Witwatersrand Supergroup directly. The palaeocurrent distribution of the Conglomerate Unit is unimodal, with modes towards the southwest in the southern parts of the study area and towards the north in the northern regions. Most of the palaeocurrent distributions of the Quartzite Unit are unimodal, too, but bimodal distributions were found at three localities. Pebble size of the Black Reef placer is largest in the northeastern parts of the study area, but no orderly lateral size variation was found. Pebble roundness, too, varies greatly and apparently randomly. The composition of the pebble assemblage is not constant, but no systematic lateral change could be detected. A petrographic study of the arenites of the Formation reveals a remarkable textural and mineralogical maturity, especially for the upper beds. It is concluded that the pre-Transvaal palaeosurface had a palaeorelief of up to 30 m and that the topography of the palaeo-landscape was the dominant factor controlling early sedimentation in the basin. The palaeo-grabens probably constituted the valleys of shallow braided stream systems that drained south-westwards and northwards from a palaeo-drainage divide in the northern parts of the study area. Sediment, including detrital gold, was derived from erosion of Witwatersrand rocks and fed to the graben valleys via several alluvial fans. During a subsequent transgression, the fluvial systems became drowned and transgressive estuarine conditions ensued. During the final stages of siliciclastic sedimentation, the upper quartzite beds of the Formation were probably reworked by shallow marine processes before carbonate precipitation commenced. The cause of the marine transgression is not known beyond doubt. It is suggested, however, that lithospheric rifting, which initiated the extrusion of the underlying Ventersdorp lavas, resumed during early Transvaal times, resulting in complete severing of the continental crust and the creation of a linear sea.     

New paleomagnetic constraints on rift basin evolution in the northern Himalaya mountains

The late Cenozoic sediments in the rift basins in the northern Himalaya Mountains document important information about the uplift and deformation of the most active tectonic region in the Tibetan Plateau. However, these sediments have not been precisely dated, hindering our ability to address the basin development and termination associated with a series of uplifts in the southern Tibetan Plateau. Here, we report a detailed magnetostratigraphic study on the fluvio - lacustrine sedimentary sequence of the Dati Formation bearing abundant Hipparion forstenae fossils in the Dati Basin in the northern frontal region of the Himalaya Mountains. The 195 m – thick section yielded six normal and seven reversed polarity zones that correlate well with Chrons C3An.1r to C4r.2r of the geomagnetic polarity time scale, constraining the section age to ~8.6 – ~6.2 Ma. Together with the magnetostratigraphic results from other rift basins in the region, these results indicate that the horizons bearing the Hipparion fossils were deposited during the age interval of 7.1–6.5 Ma in the northern Himalaya Mountains. The regional tectonic activity and comprehensive magnetostratigraphic and sedimentologic comparisons suggest that the evolution of the rift basins in the northern Himalaya Mountains has involved three major stages since the late Cenozoic, i.e., (1) ~10.0–8.0 Ma, onset of the basins with fan delta facies; (2) ~8.0–3.0 Ma, expansion of the basins with mainly lacustrine facies; (3) ~3.0–1.7 Ma, shrinking and termination of the basins with alluvial fans. The basin evolutionary history indicates an accelerated tectonic uplift of the Himalaya Mountains at ~10.0 Ma, and two deformational events at ~3.0 Ma and at ~1.7 Ma.