Global Tectonic Events Research Articles

Indian carbonatites in the global tectonic context

Chrono-tectonic settings of the carbonatite occurrences of India are reviewed with a focus on the “big picture” of carbonatite emplacements in the Indian plate in relation to global tectonic events associated with the amalgamation and breakup of supercontinents. Four chrono-tectonic domains, namely, Southern domain, Southeastern domain, Northeastern domain and Northwestern domain, are delineated based on the geographical distribution, tectonic settings and temporal relationships amongst the carbonatite complexes. The Southern domain comprises two sub-domains – Paleoproterozic and Neoproterozoic. The Paleoproterozoic sub-domain is related to extension due to relaxation after the Southern Granulite Terrain-Dharwar accretion, while the Neoproterozoic sub-domain is related to rifting related to the fragmentation of Rodinia. The Southeastern domain is related to the Mesoproterozoic fragmentation of Columbia. The Northeastern domain is related to the Mid-Cretaceous breakup of Greater India from Australia-Antarctica driven by the Kerguelen mantle plume that also produced the Rajmahal-Sylhet Large Igneous Province (LIP). The Northwestern domain is related to the Late-Cretaceous Indo-Seychelles-Madagascar split and the passage of Greater India over the reunion hotspot, which also produced the Deccan LIP.

Insights into the Crustal Evolution and Tungsten Mineralization of the West Cathaysia Block: Constraints from the Inherited Zircons from the Mesozoic Dengfuxian and Paleozoic Tanghu Plutons, South China

The formation and evolution of the ancient continental crust are crucial issues in solid-earth geology which are commonly associated with global tectonic events and the formation of economically valuable magmatic-hydrothermal ore deposits. The Cathaysia Block, one of the ancient continents in Southeast Asia, can be subdivided into two parts: the West Cathaysia Block and the East Cathaysia Block. Unlike the East Cathaysia Block, no Precambrian rocks are exposed in the West Cathaysia Block, constraining further understanding of the formation and evolution of this block. In this study, a total of four hundred and thirty-three zircon U-Pb dating analyses and two hundred and eighteen Lu-Hf isotopic analyses on zircon grains from the Jurassic Dengfuxian granites and Ordovician Tanghu granites, Nanling Range, were carried out. LA-ICP-MS zircon U-Pb dating yields mean average 206Pb/238U ages of 152.6 ± 2.2 Ma (MSWD = 1.6) and 442.4 ± 1.7 Ma (MSWD = 3.8), which are regarded as the rock-forming age for the Jurassic Dengfuxian granites and Ordovician Tanghu granites, respectively. The 207Pb/206Pb ages of the inherited zircons from the Jurassic Dengfuxian granites and Ordovician Tanghu granites range from 522 Ma to 3395 Ma, hosting two major peaks at the 0.9–1.0 Ga and 2.4–2.5 Ga. In contrast to the East Cathaysia Block, the West Cathaysia Block lacks the age peak of 1.8–1.9 Ga, indicating that the West Cathaysia Block was not influenced by the assembly of the Columbia supercontinent in the Paleo-Proterozoic. In combination with the Lu-Hf isotopes, we proposed that the crust evolution of the West Cathaysia Block in Archean is dominated by juvenile crustal growth events, and dominated by the crustal reworking since the Proterozoic. The long duration of crustal reworking in the West Cathaysia Block resulted in the enrichment of lithophile elements (e.g., W, Sn, Nb, and Ta) in the crust of that region. Therefore, the Jurassic granites in the Nanling Range, which are mainly derived from the partial melting of Proterozoic basement rocks, became associated with large-scale tungsten polymetallic mineralization.

Terrestrial impact craters track the voyage of lithospheric plates

The paradigm of plate tectonics has aided in the identification of the journey of continents on the globe, their assembly into supercontinents, disruption, and re‐assembly. Here, we use meteorite impact craters as proxies for tracking the voyage of lithospheric plates. Employing the provisions in GPlates, an interactive geographic information system‐based plate tectonic reconstruction model, we were able to identify the palaeo‐position, and velocity of the 174 terrestrial impact craters, formed after 1,100 Ma, across the globe. These parameters of craters were evaluated for independent tectonic plates and were correlated with global tectonic events. For example, the similarity in the velocity of Beaverhead (900 Ma) and Holleford (550 Ma) craters since 550 Ma is traced to the connection between the Eastern Basin and North America Craton commencing 1,100 Ma, and through the South Basin and Range. Likewise, the drastic reduction in the velocity of Spider Crater (700 Ma) in Australia after 600 Ma can be attributed to the subduction between east and west Gondwana. The accelerated motion of the Indian Plate at 63 Ma, when the lithosphere was hovering over the Réunion hotspot, is also explained. With the advent of more improved plate tectonic models and the discovery of more impact craters, improvised interpretations will be possible.

The Detection of Seismicity on Icy Ocean Worlds by Single‐Station and Small‐Aperture Seismometer Arrays

AbstractFuture missions carrying seismometer payloads to icy ocean worlds will measure global and local seismicity to determine where the ice shell is seismically active. We use two locations, a seismically active site on Gulkana Glacier, Alaska, and a more seismically quiet site on the northwestern Greenland Ice Sheet as geophysical analogs. We compare the performance of a single‐station seismometer against a small‐aperture seismic array to detect both high (>1 Hz) and low (<0.1 Hz) frequency events at each site. We created catalogs of high frequency (HF) and low frequency (LF) seismicity at each location using an automated short‐term average/long‐term average technique. We find that with a 1‐m small‐aperture seismic array, our detection rate increased (9% for Alaska and 46% for Greenland) over the single‐station approach. At Gulkana, we recorded an order of magnitude greater HF events than the Greenland site. We ascribe the HF events sources to a combination of icequakes, rockfalls, and ice‐water interactions, while very HF events are determined to result from bamboo poles that were used to secure gear. We further find that local environmental noise reduces the ability to detect LF global tectonic events. Based upon this study, we recommend that (a) future missions consider the value of the expanded capability of a small array compared to a single station, (b) design detection algorithms that can accommodate variable environmental noise, and (c) assess the potential landings sites for sources of local environmental noise that may limit detection of global events.

Sources of K-bentonites across the Ordovician-Silurian transition in South China: Implications for tectonic activities on the northern and southern margins of the South China Block

During the Ordovician-Silurian transition, continuous large-scale explosive volcanism occurred around the world, including in South China, which reflected intense global plate movement in this transition. The tectonic activities leading to volcanism in the South China Block during this transition, including the northern and southern margins, remain controversial. K-bentonite altered from volcanic ash maintains reliable records of local and global tectonic events. Accordingly, research on the sources of K-bentonite layers in South China across this transition can provide implications for disputes about tectonic activities in the South China Block. This study presents a detailed geochemical analysis of the sources of K-bentonite layers in the Wufeng Formation (Upper Ordovician Katian stage) and Longmaxi Formation (Lower Silurian Rhuddanian stage) in three sections and one well in South China. Both bulk and zircon geochemical features indicate that the K-bentonite layers in South China during this transition had two sources, implying that volcanism originated from two different locations with two different geodynamic settings in or near the South China Block. K-bentonite from a within-plate (anorogenic) tectonic setting implies that within-plate extension on the northern margin of the South China Block was initiated in the Late Ordovician. K-bentonite from an arc-related (orogenic) tectonic setting implies that subduction-collision process occurred on the southern margin of the South China Block during this transition.

Global implication of mesoproterozoic (~\u20091.4\xa0Ga) magmatism within the\xa0Sette-Daban Range (Southeast Siberia)

Mesoproterozoic period included several global tectonic events like break-up of Nuna and formation of Rodinia. However, although Siberia is a significant piece of both supercontinents, Mesoproterozoic time is marked by quiescence of magmatic and tectonic activity in it. We report here a mafic dyke (named Gornostakh dyke) in the southeastern Siberian Craton dated at 1419 ± 32 Ma by LA-ICPMS U–Pb geochronology of apatite. The dyke has tholeiitic compositions with high MgO and alkaline content, low-Ti, and arc-like trace element pattern. Due to the absence of subduction tectonics in the study area, geochemical data could be attributed to a significant contribution from metasomatically enriched subcontinental lithospheric mantle previously modified by subduction processes. That kind of composition is common for low-Ti dykes of intraplate flood basalt provinces similar to, for example, Permian–Triassic Siberian large igneous province (LIP). Paleogeographic reconstructions suggest that Siberia was connected to Laurentia and Baltica and their reconfiguration interrupts a prolonged tectonic quiescence in the Siberian Craton from ca. 1.88 Ga reflecting a transition from Nuna to Rodinia configuration. The mafic magmatism on 1419 Ma on the southeastern margin of the Siberian Craton together with coeval extensional tectonics observed in the structure of the Sette-Daban ridge proposes a hypothetical LIP which may be a direct consequence of the beginning of this transition.

40Ar/39Ar Geochronology and New Mineralogical and Geochemical Data from Lamprophyres of Chompolo Field (South Yakutia, Russia)

The alkaline igneous rocks of the Chompolo field (Aldan shield, Siberian craton), previously defined as kimberlites or lamproites, are more correctly classified as low-Ti lamprophyres. The emplacement age of the Ogonek pipe (137.8 ± 1.2 Ma) and the Aldanskaya dike (157.0 ± 1.6 Ma) was obtained using 40Ar/39Ar K-richterite dating. The Chompolo rocks contain abundant xenocrysts of mantle minerals (chromium-rich pyropic garnets, Cr-diopsides, spinels, etc.). The composition of the mantle xenocrysts indicates the predominance of spinel and garnet–spinel lherzolites, while the presence of garnet lherzolites, dunites, harzburgites, and eclogites is minor. The Chompolo rocks are characterized by large-ion lithophile element (LILE) and Light Rare Earth Element (LREE) enrichments, and high field strength element (HFSE) depletions. The rocks of the Ogonek pipe have radiogenic Sr (87Sr/86Sr (t) = 0.70775 and 0.70954), and highly unradiogenic εNd(t) (−20.03 and −20.44) isotopic composition. The trace element and isotopic characteristics of the Chompolo rocks are indicative of the involvement of subducted materials in their ancient enriched lithospheric mantle source. The Chompolo rocks were formed at the stage when the Mesozoic igneous activity was triggered by global tectonic events. The Chompolo field of alkaline magmatism is one of the few available geological objects, which provides the opportunity to investigate the subcontinental lithospheric mantle beneath the south part of the Siberian craton.

Active rifting and bimodal volcanism in Proterozoic Papaghni sub-basin, Cuddapah basin (Andhra Pradesh), India

Bimodal volcanism in the Cuddapah basin is associated with a cratonic rift setting. The Cuddapah basin consists of five sub-basins (viz., Papaghni, Nallamalai, Srisailam, Kurnool and Palnad) and a total thickness of ~12 km sediments and associated bimodal volcanics. The oval-shaped gravity high observed over the Papaghni sub-basin is due to lopolithic intrusions along listric faults. A basin evolution model is prepared in this context with signatures of active rifting. Mapping and geochemical sampling along the Tadpatri–Tonduru tract along with petrographic observations additionally supports the proposed model. The model presents the mechanism of bimodal volcanism during rifting and sedimentation. Basin evolution with tectonic modifications revealed a link with global tectonic events (e.g., ~1.8 Ga Hudsonian orogeny, ~1.3 Ga Grenville orogeny, ~0.9 Ga Enderbia docking). The stratigraphic disposition of the surge, flow, fall and volcaniclastic deposits in this old Proterozoic terrane indicate the magma history and eruption conditions. The felsic volcanic rocks are classified as rhyolite and rhyodacite. The mafic volcanics are mainly basaltic. Primordial mantle normalized trace element plots indicate enrichment of large ion lithophile elements (Rb, Th and K) along with negative Sr, P and Ti anomalies. The chondrite normalized REE patterns are characterized by LREE enrichment, negative Eu anomaly and flat HREE pattern. These features indicate origin of felsic volcanics through shallow crustal melting with plagioclase either as a residual or a fractionating phase. The mafic rocks of the area are product of shallow mantle melting related to asthenospheric upwelling followed by decompression melting and generation of basaltic magma. This was also associated with lithosphereic stretching, rifting and initiation of sedimentation. The less viscous mafic magma was probably channelized along the rift-related faults. The underplating and intraplating of hot mantle-derived magma supplied heat into the crust. The attendant partial melting of continental crust produced the felsic magma. Different sub-basins within the Cuddapah basin indicates a combined mechanism of rifting and orogenic events.

Carbon-isotope record and paleoceanographic changes prior to the OAE 1a in the Eastern Tethys: Implication for the accumulation of organic-rich sediments

Abstract The Barremian and Aptian correspond to periods of marked changes in the atmosphere-ocean system, and offer valuable information about global tectonic events. Here, we present integrated high-resolution carbonate carbon-isotope (δ13Ccarb) and geochemical data that precede the Early Aptian oceanic anoxic event (OAE 1 a) in the Qiangtang Basin (Eastern Tethys). We identified a characteristic and well-correlatable Barremian-Lower Aptian carbon-isotope pattern that can be categorized into 10 Segments. The most notable features of the δ13Ccarb record are multiple negative carbon-isotope excursions in the Barremian-Aptian, and each is possibly linked to environmental changes. Two carbon isotopic plateaus are observed in the Upper Barremian, which are generally related to increased primary productivity, and to the concomitant development of dys-to anaerobic conditions. Our data indicate the development of anoxic conditions during shale deposition, which occurred under the conditions of a semi-enclosed lagoon environment. Such conditions, were likely localized, and not the primary mechanism responsible for the deposition of organic-rich sediments within the Shenglihe succession. The deposition of organic-rich shales was very likely due to global/regional warming. Rising temperatures and hydrological cycle strengthening during shale deposition are believed to have increased primary productivity, and thus resulted in the expansion of the local oxygen minimum zone within the water column.

Duration, evolution, and implications of volcanic activity across the Ordovician–Silurian transition in the Lower Yangtze region, South China

Volcanism provides a reliable record of local and global tectonic events and substantially influences both modern and ancient environments, climates, and the evolution of life. The Ordovician–Silurian (O–S) transition is a special period because intensive volcanism occurred globally, including in the Yangtze region of South China. Volcanic events during this period are a symptom of plate tectonic behaviour and are thought to be responsible for the remarkable changes in climate in the early Palaeozoic, though the relationships between these events remain unclear and controversial. Coeval igneous rocks and volcanic sediments (VS) are primarily used to resolve this issue. However, limited studies have been performed on VS from the O–S transition in South China. Recently, a typical VS-bearing section was found in the Lower Yangtze region, which contains ∼100 thin, interbedded volcanic ash layers across the O–S transition. Detailed petrographic and geochemical analyses of the volcanic ashes were conducted to determine their isotopic ages, magma sources, evolutionary processes, and tectonic settings. Our preliminary results suggest that volcanic eruptions in South China lasted for more than 22 Ma across the O–S boundary, from ∼449.3 ± 3.6 to 427.6 ± 4.1 Ma, where 445.14 Ma is the lowermost graptolite biozone for Metabolograptus extraordinarius, as well as the initiation of the Late Ordovician mass extinction (LOME) event in the Yangtze region. The evolutionary history of the parental magma was constructed from a depleted mantle source in the early stage and from a crustal source in the late stage, with several transitional features in the middle. The mantle source and arc-related geochemical indicators for the volcanic ashes support the disputed “subduction-collision orogeny” model. We propose that the strong volcanism in South China, accompanied by volcanism in numerous other regions worldwide, was an important trigger for the LOME and was likely responsible for oceanic 87Sr/ 86Sr fractionation and other climatic changes during the O–S transition.

Discussion on the relationship between the Yanshanian Movement and cratonic destruction in North China

The relationship between the Yanshanian Movement, destruction of the North China Craton (NCC), and subduction of the western Pacific plate is crucial to reconstructing the middle-late Mesozoic tectonic evolution of the eastern Asian continent and margin. The Yanshanian Movement was a globally important change in crustal tectonics during the Middle-Late Jurassic. Previous research has systematically studied the formation and evolution of the Yanshanian Movement, focusing on the timing and location of tectonic movements, and the sedimentary and volcanic strata. However, the question of whether the tectonic activity occurred globally, and the characteristics of the Yanshanian Movement remain debated. The main argument is that if a tectonic movement can only be characterized by a regional or local disconformity, and if the tectonic movement occurred in an intracontinental setting, with extensive deformation but with no disconformity despite volcanic eruptions and magmatic intrusions, accompanied by changes in crustal structure and composition, should it be defined as a tectonic event or process? This question requires further analysis. The main aim of this study is to distinguish whether the Yanshanian Movement is a local feature of the eastern Asian continent, or a global tectonic event related to subduction of the Pacific Plate. In this paper, based on previous research, we discuss the spatial and temporal evolution of the Yanshanian Movement, the controlling tectonic mechanisms, and its relationship to the reactivation and destruction of the NCC and the subduction of the western Paleo-Pacific slab. We emphasize that the Yanshanian Movement in the Middle-Late Jurassic is distinct from the lithospheric thinning responsible for Early Cretaceous extension and magmatism related to the destruction of the NCC. The various tectonic stages were constrained by different dynamics and tectonic settings, or by different tectonic events and processes. Therefore, it is possible that the deformation and reactivation of the NCC contributed to its destruction, in addition to lithospheric thinning. Finally, we discuss whether the Yanshanian Movement was associated with the destruction of the NCC.

Petrogenesis, detrital zircon SHRIMP U-Pb geochronology, and tectonic implications of the Upper Paleoproterozoic Seosan iron formation, western Gyeonggi Massif, Korea

This study involves investigations on the Upper Paleoproterozoic iron formation (viz., Seosan iron formation) from the Seosan Group, Gyeonggi Massif of the southwestern Korean Peninsula. It occurs as thin banded layers within meta-arkosic sandstone, formed by alternating processes of chemical (hydrothermal) and detrital depositions under a shallow marine environment. It mainly consists of alternating layers of iron oxides, mostly hematite, and quartz. Minor amounts of magnetite surrounded by muscovite, clinopyroxene and amphibole indicate hydrothermal alteration since its formation. Meta-arkosic sandstone is composed of recrystallized or porphyroclastic quartz and microcline, with small amounts of hematite and pyrite clusters. The Seosan iron formation has high contents of total Fe2O3 and SiO2 with positive Eu anomalies similar to those of other Precambrian banded iron formations, and its formation is clearly related to hydrothermal alteration since its deposition. Detrital zircon SHRIMP U-Pb geochronology data from a meta-arkosic sandstone (SN-1) and an iron formation (SN-2) show mainly two age groups of ca. 2.5 Ga and ca. 1.9–1.75 Ga. This together with intrusion age of the granite gneiss (ca. 1.70–1.65 Ga) clearly indicate that the iron formations were deposited during the Upper Paleoproterozoic. The dominant Paleoproterozoic detrital zircon bimodal age peaks preserved in the Seosan iron formation compare well with those from the South China Craton sedimentary basins, reflecting global tectonic events related to the Columbia supercontinent in East Asia.

Ocean Basin Evolution and Global-Scale Plate Reorganization Events Since Pangea Breakup

We present a revised global plate motion model with continuously closing plate boundaries ranging from the Triassic at 230 Ma to the present day, assess differences among alternative absolute plate motion models, and review global tectonic events. Relatively high mean absolute plate motion rates of approximately 9–10 cm yr−1 between 140 and 120 Ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time. An event at ∼100 Ma is most clearly expressed in the Indian Ocean and may reflect the initiation of Andean-style subduction along southern continental Eurasia, whereas an acceleration at ∼80 Ma of mean rates from 6 to 8 cm yr−1 reflects the initial northward acceleration of India and simultaneous speedups of plates in the Pacific. An event at ∼50 Ma expressed in relative, and some absolute, plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4–5 cm yr−1 indicates that an increase in collisional forces (such as the India–Eurasia collision) and ridge subduction events in the Pacific (such as the Izanagi–Pacific Ridge) play a significant role in modulating plate velocities.

Reconstruction of Paleoceanography Significance in the Western Pacific and Atlantic Oceans during the Neogene Based on Calcareous Nannofossil Productivity and Size Variations, Related to the Global Tectonic Events

We investigated the calcareous nannofossils in the western Pacific and Bahama Bank of Caribbean Sea to reconstruct the paleoceanography and correlate with the global events. The absolute abundant of coccolith (number/g) are gradually increased from NN6 throughout NN19 Zone, while the relative abundance of Discoaster is decreased at Sites 782 in the western Pacific Ocean. The changes of the modal and maximum size of Reticulofenestra which are strongly reflected the collapse of sea surface stability, show four times in 8.8 Ma, 6.4 Ma, 5.4 Ma and 3.75 Ma at Site 782. On the basis of relationship between the changes of maximum sizes of Reticulofenestra and nutrient condition, these eutrophication events are clearly traceable to the western Pacific, Bahama Bank, northwestern Pacific Ocean and to the Indian Ocean. Two paleoceanographic events found in 8.8 Ma and 3.75 Ma are interpreted as change to high nutrient condition resulted in the intensification of Asian Monsoon and closure of Panama Isthmus.

Inversion of calcite twin data, paleostress reconstruction and multiphase weak deformation in cratonic interior – Evidence from the Proterozoic Cuddapah basin, India

Paleostress orientations from mechanically twinned calcite in carbonate rocks and veins in the neighborhood of large faults were investigated to comment on the nature of weak upper crustal stresses affecting sedimentary successions within the Proterozoic Cuddapah basin, India. Application of Turner's P–B–T method and Spang's Numerical dynamic analysis on Cuddapah samples provided paleostress orientations comparable to those derived from fault-slip inversion. Results from the neighborhood of E–W faults cutting through the Paleoproterozoic Papaghni and Chitravati groups and the Neoproterozoic Kurnool Group in the western Cuddapah basin, reveal existence of multiple deformation events − (1) NE–SW σ3 in strike-slip to extensional regime along with an additional event having NW–SE σ3, for lower Cuddapah samples; (2) compressional/transpressional event with ESE–WNW or NNE–SSW σ1 mainly from younger Kurnool samples.Integrating results from calcite twin data inversion, fault-slip analysis and regional geology we propose that late Mesoproterozoic crustal extension led to initial opening of the Kurnool sub-basin, subsequently influenced by weak compressional deformation. The dynamic analysis of calcite twins thus constrains the stress regimes influencing basin initiation in the southern Indian cratonic interior and subsequent basin inversion in relation to craton margin mobile belts and plausible global tectonic events in the Proterozoic.

The Sr isotope chemostratigraphy as a tool for solving stratigraphic problems of the Upper Proterozoic (Riphean and Vendian)

Published and original data on the Sr isotopic characterization of carbonates from the Riphean and Vendian key sections of the Southern Urals, Siberia, Asia, Africa, Australia, and North America are considered in compliance with the suggested principles of reconstructing the Sr isotopic composition of the Proterozoic seawater. The suggested methodic approach is used to plot the reference curve of the 87Sr/86Sr variations in the Riphean and Vendian oceans. During the time span of 1600–1250 Ma, the 87Sr/86Sr variations were in a narrow range corresponding to 0.70456–0.70494, but approaching the date of about 1030 Ma, the 87Sr/86Sr ratio rose to 0.70601–0.70611 and then quickly declined to 0.70519–0.70523 near the date of 1000 Ma. In the second half of the late Riphean and in the Vendian, the ratio grew almost steadily from 0.70521–0.70535 to values of 0.70874–0.70885 characteristic of the Late Vendian time. The subsequent regular growth of that ratio in seawater lasted from 840 to 550 Ma, though there were short-term epochs when the ratio noticeably dropped to 0.70561–0.70575 at approximately 760 Ma and to 0.70533–0.70538 at 670–660 Ma. After the mid-Late Vendian maximum, it declined to 0.70812–0.70823 at the end of the Nemakit-Daldynian Age and decreased to 0.70806–0.70812 during the Tommotian Age of the Early Cambrian. As is shown, the Sr isotopic variations in the Riphean and Vendian oceans were interrelated with global tectonic events in geospheres and formation stages of the Rodinia and Gondwana supercontinents. The Baikalian Complex of Siberia is considered in the work as a case in point illustrating advantages of the expounded approach with respect to age substantiation of particular stratigraphic subdivisions.

The coupling relationship between the west Shandong rise and the Jiyang depression, China

This article reports 21 AFT (apatite fission track) data from the West Shandong (山东) rise (WSR) and Jiyang (济阳) depression, and mainly studies their Cenozoic uplifting/subsidence history and the relationship between them. Furthermore, we improve our insights into the Bohai Bay Basin (BBB). Our AFT analysis and AFT T-t modeling indicates that the WSR was uplifted at ca. 65 Ma with apparent uplift rate of 0.019 mm/a; it underwent two relatively rapid uplifting events at 43–33 and 16–0 Ma with rates of 0.097 and 0.052 mm/a, respectively. Meanwhile, the Jiyang depression subsided at rate of 0.032 mm/a at 52–43 Ma, and the rate increased to 0.13 mm/a at ca. 42–33 Ma; finally the subsidence rate increased to 0.053 mm/a in 16–0 Ma. They all underwent a uplift in time of 23–16 Ma with rate of 0.04–0.07 mm/a. A careful comparison shows that the Cenozoic uplifting of the WSR coupled well with the subsidence of the Jiyang depression. Our research also suggests that the uplift-basin coupling events are part of the couplings between the Bohai Bay Basin and its peripheral mountains. This intraplate mountain-basin coupling is a reflection of global tectonic events.

Tectonics or climate: What drove the Miocene global expansion of C4 grasslands?

Investigations into the most plausible causes which triggered the Late Miocene global expansion of C4 grasslands have reached no consensus and are still being debated. The global decline in CO2 levels in the Late Miocene has been thought to be the most common driver. Although C4 grassland expansion was largely confined to the Late Miocene, their first appearance varied significantly in different parts of the Old and New world and therefore emphasizes role of local and/or regional factors may or may not be in combination with one or more global factors. The dynamic Himalayan orogen constitutes a significant global tectonic event that is believed to have affected global climate. Hence, a study of the Himalayan foreland basin sediments could help in gleaning out possible causes behind this major paleoecologic event, which could perhaps be directly or indirectly related. Stable carbon isotope analyses of a total of 141 pedogenic carbonates in the Mio-Pleistocene Siwalik paleosols of the Ramnagar sub-basin are coherent, providing a better insight into paleovegetational changes across the sub-basin on temporal and spatial scales. Furthermore, paleovegetational history during Oligocene is reconstructed from Dagshai paleosols exposed in the Subathu sub-basin. The isotopic results show dominance of C3 vegetation pre-7 Ma and dominance of C4 vegetation post-5 Ma. Percentage abundance of C4 vegetation was less than 20 % pre-7 Ma but increased to more than 40 % post-5 Ma, reaching up to 100 % in the youngest analyzed sediments. There is exclusive dominance of C3 vegetation during Oligocene. These results conform to the pattern of change in vegetation documented in other parts of the Himalayan belt. The global expansion of C4 grasslands largely during Late Miocene have long been linked with then climate changes, particularly brought by declining atmospheric CO2 level, large-scale fires, intensification of monsoon, seasonality, and aridity as demonstrated by various researchers. These major hypotheses explaining expansion of C4 grasslands during Late Miocene are not very convincing due to significant shortcomings associated with them, for example, expansion of C4 grasslands in non-monsoonal regions put serious constraints on the monsoonal intensification hypothesis. Likewise, aridity as being the primary driver of C4 grassland expansion seems hard to support since there is no substantial evidence that could suggest global onset of arid conditions at about same geologic time, and some most arid periods, like the Triassic, show no evidence of C4 plants. We suggest that the initial lowering of CO2 below 450 ppmV created an environment for the beginning of C4 vegetation, but the persistence of this threshold value for a considerable time during Late Miocene appears to be the probable cause of the Late Miocene global expansion of C4 grasslands irrespective of their time of first appearance. It has been deduced through the correlation of Himalayan tectonic events with atmospheric CO2 levels and paleovegetational changes since Upper Miocene times that it was indeed the Late Miocene continuous, intense tectonic instability of the Himalayas that significantly decreased atmospheric CO2 levels and which perhaps played a key role in changing the nature of photosynthetic pathways.

Palaeoproterozoic of India: an introduction

A relatively rapid change in the Earth’s surface processes has been anticipated across the Archaean– Palaeoproterozoic boundary as a consequence of changes in the crust–mantle system and tectonic regime (Condie 1989, 1997; Eriksson et al. 2004; Reddy & Evans 2009). The Palaeoproterozoic era (2500–1600 Ma: Plumb 1991) represented perhaps the first supercontinent cycle, from the amalgamation and dispersal of a Neoarchaean supercontinent to the formation of the 1.9–1.8 Ga supercontinent Nuna (Reddy & Evans 2009), and encompasses one or more global tectonic event that coincides with fundamental changes in the integrated system of core, mantle, lithosphere, hydrosphere, atmosphere and biosphere (i.e. an integrated Earth System). An integration of seemingly disparate geoscience disciplines is therefore an essential prerequisite to understand these changes (Reddy & Evans 2009); and that was the aim of the UNESCOIGCP 509 project (2005–2009) on Palaeoproterozoic Supercontinents and Global Evolution. The fifth and final conference and postconference field workshop (in the Singhbhum Craton) related to the UNESCO-IGCP 509 project was organized by the Indian Statistical Institute (ISI), Kolkata with financial support from the UNESCO, the ISI, and the Council of Scientific and Industrial Research, Government of India during the period 26 October–3 November 2009. An entire session was devoted to the Palaeoproterozoic geology of India, as the Indian Shield represents a vast repository of the Palaeoproterozoic geological record. While most of the papers presented in this session were essentially on Palaeoproterozoic geology of the Indian Shield, a few were on the geological record during the Archaean– Palaeoproterozoic and Palaeo-Mesoproterozoic transitions. Some clues to the tectonic regime changes during these transitions also add to our understanding of the Palaeoproterozoic Earth processes, and how they connect with earlier or later periods of Earth’s evolutionary history. A thematic set of 13 papers on various regional as well as broader issues of the Indian Palaeoproterozoic geology and their global context constitute the present Special Publication. Built over the four major amalgamated Archaean nucleii, the major and minor Palaeoproterozoic sedimentary basins and supracrustal sequences in India contain potential information comparable only in scales to those of North America, Africa, Australia and Brazil. The deformation of these supracrustal sequences, attendant metamorphism and emplacement of plutonic bodies hold important clues to their connection with major orogenies, and facilitate investigations into global correlations. The latter has a direct bearing on refining models of Palaeoproterozoic supercontinent assembly and break-up. This volume begins with an overview of the Palaeoproterozoic geological records of the Indian Shield by Saha & Mazumder, wherein the key stratigraphic and tectonic issues are highlighted. Major Palaeoproterozoic sedimentary systems, according to these authors, developed in an intracratonic to marginal-marine setting. In significant contrast to the presence of banded iron formations (BIFs) in the Archaean of Singhbhum and Dharwar, the Indian Palaeoproterozoic supracrustals are devoid of BIFs. This article also provides a brief overview of the fold belts that straddle the suture between major Archaean nuclei, and the attendant tectonic processes. A comprehensive tectonic model for the amalgamation of the major Archaean nuclei of Dharwar, Bastar, Singhbhum and Aravalli–Bundelkhand, according to these authors, is yet to emerge and is of paramount importance to further our understanding of Palaeoproterozoic supercontinent development. An updated critical analysis of the Mesoarchaean–Palaeoproterozoic stratigraphic record of the Singhbhum Craton and a synthesis thereof has been undertaken by Mazumder et al. According to these authors, the stratigraphic record of the Singhbhum Craton across the Archaean–Palaeoproterozoic boundary implies a changing tectonic scenario. They have pointed out problematic and controversial regional stratigraphic issues that need to be resolved through further geochronological studies in order to understand the relationship between Singhbhum and other cratonic provinces of India, Australia and South Africa. In a complementary paper, Mazumder et al. have presented a critical analysis of the Singhbhum Palaeoproterozoic supracrustals, and compared the northern and southern Singhbhum successions, and their possible relationship with global-scale events and those identified on Kaapvaal. Abbott et al. have reported native Fe, native Si and (Fe, Mn)S

Evaluating the gas potential of Cooper Basin Permian lacustrine mudrocks: a comparison with North American marine shale gas reservoir

The methods to evaluate the potential of marine shale as gas producers have been refined during the past five years with the development of new logging tools and increased experience gained from activity in the US. While most shale plays in the US were deposited in a marine environment, in Australia, however, extensive shales were deposited in continental basins. The question is: are continental and marine shales similar enough for the same methodology to be applied—if not, how should it be adapted or changed? The type of organic matter, its abundance and the type of hydrocarbons it can generate may differ significantly between continental shale and marine shale. Long-range lateral continuity can be expected from marine deposits as water bodies are commonly wide and deposition processes are well known. Conversely, in continental environments water body sizes are more variable and usually smaller. In a marine environment, the formations containing organic matter are relatively easy to identify because accommodation depends on global events (eustatic variations, global tectonic events). In continental formations it is more difficult, as accommodation is strongly dependent on regional tectonics. In this extended abstract, a case from the Woodford shale (US) is compared to a case from the Cooper Basin (Australia).Differences in mineralogy, type, abundance and maturity of the organic matter are analysed and their impact on the log response is discussed. Finally, recommendations are made to adapt the methodology to continental shales.