2020 Vol. 11, No. 5

Research Paper
High-resolution patterns of palaeoenvironmental changes during the Little Ice Age and the Medieval Climate Anomaly in the northwestern Iberian Peninsula
Daniel Castro, Martín Souto, María Isabel Fraga, Eduardo García-Rodeja, Sebasti an P erez-Díaz, Jos e Antonio L opez S aez, Xabier Pontevedra-Pombal
2020, (5): 1461-1475. doi: 10.1016/j.gsf.2020.05.015
Abstract(199) HTML PDF(13)
A high resolution core (9.7 yr cm1) from the Chao de Veiga Mol raised bog (NW Iberian Peninsula) was analyzed to identify plant macrofossils, estimate peat humification and calculate hydroclimatic indices based on current bog species, with the overall aim of determining the climate conditions associated with evolution of the bog during the Medieval Climate Anomaly and the Little Ice Age. These proxies, together with historical and climate data, proved to be good indicators of the changes in bog surface wetness. Analysis: of the core led to identification of 9 different periods: two corresponding to the so-called Medieval Climate Anomaly (930 to 1345 AD, 1075–665 calibrated years before present [cal. yr BP]); four corresponding to the Little Ice Age (1345 to 1905 AD; 665–105 cal yr BP); and three corresponding to the last century (1905 to 2000 AD). The findings revealed a generally dry climate that lasted until the 14th century, followed by a transition to a long period with a more humid, but characteristically very variable climate, which ended at the beginning of the 20th century and was followed by a rapid transition to more humid conditions and finally, a change to drier conditions. The Medieval Climate Anomaly was indicated by the abundance of dry-adapted mosses (Leucobryum glaucum, Hypnum cupressiforme) and characterized by warm dry conditions and high levels of peat humification, with alternating wet phases. The LIA period was dated by a large abundance of Sphagnum species (an indicator of wetness) and a gradual increase in the humification index. However, four different climate phases were differentiated in this period. High-resolution reconstruction of the evolution of the CVM bog and the multiproxy approach have together enabled a more detailed identification of climatic variations in this area, which are generally consistent with the global models, as well as better definition of the elusive climatic oscillations in the last millennium and confirmation of the importance of local modulation of global models. The study provides new information and a detailed chronology of climatic events that will help to refine local modulation of the climate evolution model in the still quite unexplored region of the NW Iberian Peninsula, a key area for understanding the paleoclimatic dynamics in SW Europe.
Recognition of two contrasting structural- and mineralogical-gold mineral systems in the Youjiang basin, China-Vietnam: Orogenic gold in the south and Carlin-type in the north
Lin Yang, Jun Deng, David I. Groves, Qingfei Wang, Liang Zhang, Wei Wu, Kai Qin, Qizuan Zhang
2020, (5): 1477-1494. doi: 10.1016/j.gsf.2020.05.011
Abstract(382) HTML PDF(26)
The gold deposits in the Youjiang basin, totaling > 25 Moz gold, have traditionally been thought to be of Carlintype, particularly those with extensional structural geometries in the northern basin dominated by platform sedimentary sequences. However, the structural geometries, mineralization styles and alteration types for the Jinya, Gaolong and Nakuang gold deposits in the south-central part of the basin are remarkably similar to those of unequivocal orogenic gold deposits. Structural studies show that gold mineralization in the three gold deposits was controlled by tight “locked-up” anticlines with NWSE- to EW-trending and/or concomitant thrusts and/or shear zones, which resulted from NESW- to N–S-trending compression or transpression following the Early Triassic closure of the Paleo-Tethyan Ocean. Alteration zones in these deposits are dominated by silicification (quartz), sericitization, sulfidation and carbonation. Zoned pyrites in these deposits comprise Au-poor cores and invisible Au-bearing rims with minor external free gold. Euhedral to subhedral auriferous arsenopyrites also contribute to the gold budget. These features indicate that the three gold deposits are sediment-hosted orogenic gold deposits that contrast markedly with the Carlin-type gold deposits in the northern part of the Youjiang basin in terms of structural geometry and timing, mineralization style and nature of associated alteration. Although additional reliable ages using robust methodologies are still required, the older isotopic ages of the gold deposits in the south-central Youjiang basin are also consistent with earlier formation during transpression that predated extension during orogenic collapse, the period of formation of the Carlin-type gold deposits in the northern Youjiang basin.
A missing link of the Proto-Tethys Ocean between the Qinling and Qilian orogens, China: Insights from geochronology and structural geology
Shujuan Zhao, Sanzhong Li, Huahua Cao, Xiyao Li, Xin Liu, Shengyao Yu, Xiaoyu Guo
2020, (5): 1495-1509. doi: 10.1016/j.gsf.2020.05.009
Abstract(403) HTML PDF(22)
The Qinling-Qilian connection zone (QQCZ) is a key area to reveal the relationship and to make a link of the North Qinling and the North Qilian orogens, China. Here we present U–Pb dating data of detrital zircons from four sedimentary/metasedimentary rocks in the QQCZ and the southwestern North China Block (NCB) and detailed regional structural data. Three episodes of fold deformation (D1, D2 and D3) are distinguished in the QQCZ, with the former two occurred during the early Paleozoic. The D1 deformation is mainly characterized by regionally penetrative schistosity and some residual rootless intrafolial folds due to the intensive superpositions by the subsequent D2 and D3 deformations. The D2 deformation characterized by tight folds, associated axial plane foliations and crenulation lineations indicates a stress field characterized by NNE–SSW-directed compression, which may be induced by the collision between the NCB and the southern blocks. The D3 deformation which might occur during the Mesozoic is marked by upright open folds and kink bands. The similarity of the detrital zircon age spectra of the Huluhe Group in the North Qilian Orogen and the Erlangping Group in the North Qinling Orogen suggests that the two groups have similar provenance, which may indicate that the North Qilian Orogen corresponded to the North Qinling Orogen in a regional tectonic framework. In addition, the remarkable age peak at ~435 Ma of the detrital zircon age spectrum of the Duanjiaxia Formation in the southwestern NCB indicates that this formation obtained the provenance of the North Qilian and North Qinling orogens, which may be generated by the collage of the southwestern NCB and the QQCZ during the Late Ordovician–Early Silurian. Based on structural, detrital zircon and metamorphic data, we suggest that the North Qilian and North Qinling orogens underwent similar evolution during the early Paleozoic due to the closure of the North Qilian and the Kuanping oceans which located at the northern boundary of the Proto-Tethys Ocean.
A novel type of neural networks for feature engineering of geological data: Case studies of coal and gas hydrate-bearing sediments
Lishuai Jiang, Yang Zhao, Naser Golsanami, Lianjun Chen, Weichao Yan
2020, (5): 1511-1531. doi: 10.1016/j.gsf.2020.04.016
Abstract(104) HTML PDF(7)
The nature of the measured data varies among different disciplines of geosciences. In rock engineering, features of data play a leading role in determining the feasible methods of its proper manipulation. The present study focuses on resolving one of the major deficiencies of conventional neural networks (NNs) in dealing with rock engineering data. Herein, since the samples are obtained from hundreds of meters below the surface with the utmost difficulty, the number of samples is always limited. Meanwhile, the experimental analysis of these samples may result in many repetitive values and 0s. However, conventional neural networks are incapable of making robust models in the presence of such data. On the other hand, these networks strongly depend on the initial weights and bias values for making reliable predictions. With this in mind, the current research introduces a novel kind of neural network processing framework for the geological that does not suffer from the limitations of the conventional NNs. The introduced single-data-based feature engineering network extracts all the information wrapped in every single data point without being affected by the other points. This method, being completely different from the conventional NNs, re-arranges all the basic elements of the neuron model into a new structure. Therefore, its mathematical calculations were performed from the very beginning. Moreover, the corresponding programming codes were developed in MATLAB and Python since they could not be found in any common programming software at the time being. This new kind of network was first evaluated through computer-based simulations of rock cracks in the 3DEC environment. After the model’s reliability was confirmed, it was adopted in two case studies for estimating respectively tensile strength and shear strength of real rock samples. These samples were coal core samples from the Southern Qinshui Basin of China, and gas hydrate-bearing sediment (GHBS) samples from the Nankai Trough of Japan. The coal samples used in the experiments underwent nuclear magnetic resonance (NMR) measurements, and Scanning Electron Microscopy (SEM) imaging to investigate their original micro and macro fractures. Once done with these experiments, measurement of the rock mechanical properties, including tensile strength, was performed using a rock mechanical test system. However, the shear strength of GHBS samples was acquired through triaxial and direct shear tests. According to the obtained result, the new network structure outperformed the conventional neural networks in both cases of simulation-based and case study estimations of the tensile and shear strength. Even though the proposed approach of the current study originally aimed at resolving the issue of having a limited dataset, its unique properties would also be applied to larger datasets from other subsurface measurements.
Detrital zircon constraints on late Paleozoic tectonism of the Bogda region (NW China) in the southern Central Asian Orogenic Belt
Qian Wang, Guochun Zhao, Yigui Han, Jinlong Yao
2020, (5): 1533-1548. doi: 10.1016/j.gsf.2020.04.002
Abstract(186) HTML PDF(9)
The Chinese North Tianshan (CNTS) in the southern part of the Central Asian Orogenic Belt (CAOB) has undergone multistage accretion-collision processes during Paleozoic time, which remain controversial. This study addresses this issue by tracing the provenance of Late Paleozoic sedimentary successions from the Bogda Mountain in the eastern CNTS through U–Pb dating and Lu–Hf isotopic analyses of detrital zircons. New detrital zircon U–Pb ages (N ¼ 519) from seven samples range from 261  4 Ma to 2827  32 Ma. The most prominent age peak is at 313 Ma and subordinate ages vary from 441 Ma to 601 Ma, with some Precambrian detrital zircon ages (~7%) lasting from 694 Ma to 1024 Ma. The youngest age components in each sample yielded weighted mean ages ranging from 272  9 Ma to 288  5 Ma, representing the maximum depositional ages. These and literature data indicate that some previously-assumed “Carboniferous” strata in the Bogda area were deposited in the Early Permian, including the Qijiaojing, Julideneng, Shaleisaierke, Yangbulake, Shamaershayi, Liushugou, Qijiagou, and Aoertu formations. The low maturity of the sandstones, zircon morphology and provenance analyses indicate a proximal sedimentation probably sourced from the East Junggar Arc and the Harlik-Dananhu Arc in the CNTS. The minor Precambrian detrital zircons are interpreted as recycled materials from the older strata in the Harlik-Dananhu Arc. Zircon εHf(t) values have increased since ~408 Ma, probably reflecting a tectonic transition from regional compression to extension. This event might correspond to the opening of the Bogda intraarc/ back arc rift basin, possibly resulting from a slab rollback during the northward subduction of the North Tianshan Ocean. A decrease of zircon εHf(t) values at ~300 Ma was likely caused by the cessation of oceanic subduction and subsequent collision, which implies that the North Tianshan Ocean closed at the end of the Late Carboniferous.
Subaerial hot springs and near-surface hydrothermal mineral systems past and present, and possible extraterrestrial analogues
Franco Pirajno
2020, (5): 1549-1569. doi: 10.1016/j.gsf.2020.04.001
Abstract(100) HTML PDF(1)
The principal features, geotectonic settings and association with near-surface hydrothermal mineral systems of ancient and present subaerial hot springs, fumaroles and geysers are herein reviewed. Fumaroles and geysers usually occur in volcanic craters and are in most cases, part and parcel of hot spring environments. Subaerial hot springs are characterised by siliceous- and carbonate-rich chemical sediments, such as sinters and travertines, respectively. Sinters are commonly enriched in various metalliferous elements. Hot springs surface discharges are also characterised by pools, which exhibit bright colours due to the presence of microorganisms. Present-day examples discussed in this paper, include the fumaroles and hot springs of the White Island volcano (New Zealand), the world-renowned Yellowstone caldera (USA) and the Afar region of the East African Rift System. The Afar triangle, in the northern part of East African Rift System, provides a good example of hot springs associated with evaporative deposits. The Tuli-Sabi-Lebombo triple junction rifts were formed during the ~ 180 Ma Karoo igneous event in southern Africa, of which the Tuli arm is the failed rift (aulacogen), as is the Afar region. The Tuli rift is effectively an unusual, if not unique, tectono-thermal setting, because it comprises Karoo-age hot springs systems and associated vein stockworks and breccias, which includes the Messina Cu deposit as well as currently active hot springs with sinter deposits probably due to post-Karoo uplift. Fumarolic pipes in ignimbrites of the Erongo Volcano-Plutonic Complex in Namibia are discussed. This is followed by an example of banded chert rocks in the Killara Formation of the Palaeoproterozoic Capricorn Orogen (Western Australia), interpreted as hot spring chemical sediments, which also show evidence of “fossil” microbial filaments. The paper ends with a brief overview of possible analogues of hot springs on planet Mars.
Plume interaction and mantle heterogeneity: A geochemical perspective
Zhen Zhang, Sanzhong Li, Guangzeng Wang, Xiyao Li, Gang Wang, Yanhui Suo, M. Santosh, Lingli Guo
2020, (5): 1571-1579. doi: 10.1016/j.gsf.2020.02.009
Abstract(158) HTML PDF(2)
Mantle plumes originating from the Core-Mantle Boundary (CMB) or the Mantle Transition Zone (MTZ) play an important role in material transfer through Earth’s interior. The hotspot-related plumes originate through different mechanisms and have diverse processes of material transfer. Both the Morganian plumes and large low shear wave velocity provinces (LLSVPs) are derived from the D00 layer in the CMB, whereas the Andersonian plumes originate from the upper mantle. All plumes have a plume head at the Moho, although the LLSVPs have an additional plume head at the MTZ. We compare the geochemical characteristics of various plumes in an attempt to evaluate the material exchange between the plumes and mantle layers. The D00 layer, the LLSVPs and the Morganian plumes are consisted of subducted slab and post-perovskite from the lower mantle. Bridgmanite would crystallize during the upwelling process of the LLSVPs and the Morganian plumes in the lower mantle, and the residual is a basalt-trachyte suite. Unlike the Morganian plumes, the crystallization in the LLSVPs is insufficient that material accumulates beneath the MTZ to form a plume head. Typically, the secondary plumes above the plume head occur at the edge of the LLSVPs because it is easier for bridgmanite crystal separating from the plume head at the edge, and the residual material with low density upwells to form the secondary plumes. Meanwhile, Na and K are enriched during the long-term crystallization process, and then the basalt-phonolite suite appears in the LLSVPs. The geochemical characteristics of Andersonian plumes suggest that the basalt-rhyolite suite is the major component in the upper mantle. Meanwhile the basalt-rhyolite suite also appears in the LLSVPs and the Morganian plumes because of the assimilation and contamination in the plume head beneath the Moho.
Timing of carbonatite-hosted U-polymetallic mineralization in the supergiant Huayangchuan deposit, Qinling Orogen: Constraints from titanite U–Pb and molybdenite Re–Os dating
Hui Zheng, Huayong Chen, Dengfeng Li, Chao Wu, Xi Chen, Chun-kit Lai
2020, (5): 1581-1592. doi: 10.1016/j.gsf.2020.03.001
Abstract(146) HTML PDF(10)
The newly-discovered supergiant Huayangchuan uranium (U)-polymetallic (Sr, Se, REEs, Ba, Nb and Pb) deposit is located in the Qinling Orogen, central China. The deposit underwent multistage mineralization, with the main carbonatite ore stage being the most important for the U, Nb, REE, Sr and Ba endowments. According to the mineral assemblages, the main carbonatite ore stage can be divided into three substages, i.e., sulfate (Ba–Sr), alkali-rich U and REE-U mineralization. Main-stage titanite from the Huayangchuan igneous carbonatite are rich in high field strength elements (HFSEs, e.g., Zr, Nb and REEs), and show clear elemental substitutions (e.g., Ti vs. Nb þ Fe þ Al and Ca þ Ti vs. Fe þ Al þ REE). High-precision LA-ICP-MS titanite dating yielded a U–Pb age of 209.0  2.9 Ma, which represents the mainstage mineralization age at Huayangchuan, and is coeval with the local carbonatite dyke intrusion. This mineralization age is further constrained by the Re–Os dating of molybdenite from the Huayangchuan carbonatite, which yielded a weighted mean age of 196.8  2.4 Ma. Molybdenite Re contents (337.55–392.75 ppm) and C-OSr- Nd-Pb isotopic evidence of the Huayangchuan carbonatite both suggest a mantle origin for the carbonatite. Our study supports that the Late Triassic carbonatite magmatism was responsible for the world-class U-Mo-REE mineralization in the Qinling Orogen, and that the regional magmatism and ore formation was likely caused by the closure of the Mianlue ocean and the subsequent North China-South China continent-continent collision.
The giant tin polymetallic mineralization in southwest China: Integrated geochemical and isotopic constraints and implications for Cretaceous tectonomagmatic event
Yanbin Liu, Lifei Zhang, Xuanxue Mo, M. Santosh, Guochen Dong, Hongying Zhou
2020, (5): 1593-1608. doi: 10.1016/j.gsf.2020.01.007
Abstract(133) HTML PDF(3)
The Gejiu-Bozushan-Laojunshan W–Sn polymetallic metallogenic belt (GBLB) in southeast Yunnan Province is an important part of the southwestern Yangtze Block in South China. Tin polymetallic mineralization in this belt includes the Niusipo, Malage, Songshujiao, Laochang and Kafang ore fields in the Gejiu area which are spatially and temporally associated with the Kafang - Laochang and Songshujiao granite plutons. These granites are characterized by variable A/CNK values (mostly > 1.1, except for two samples with 1.09), high contents of SiO2 (74.38–76.84 wt.%) and Al2O3 (12.46–14.05 wt.%) and variable CaO/Na2O ratios (0.2–0.65) as well as high zircon δ18O values (7.74‰–9.86‰), indicative of S-type affinities. These rocks are depleted in Rb, Th, U, Ti, LREE [(La/Yb)N ¼ 1.4–20.51], Ba, Nb, Sr, and Ti and display strong negative Eu and Ba anomalies. The rocks possess high Rb/Sr and Rb/Ba ratios, relatively low initial 87Sr/86Sr ratios (0.6917–0.7101), and less radiogenic εNd(t) values (8.0 to 9.1). The zircon grains from these rocks show negative εHf(t) values in the range of 3.7 to 9.9 with mean TDM2 (Nd) and TDM2 (Hf) values of 1.57 Ga and 1.55 Ga. They show initial 207Pb/204Pb ranging from 15.69 to 15.71 and 206Pb/204Pb from 18.36 to 18.70. Monazite from Songshujiao granites exhibits higher U and lower Th/U ratios, lower δ18O values and higher εHf(t) values than those of the zircon grains in the Kafang - Laochang granites. The geochemical and isotopic features indicate that the Laochang - Kafang granites originated by partial melting of Mesoproterozoic crustal components including biotite-rich metapelite and metagraywacke, whereas the Songshujiao granites were derived from Mesoproterozoic muscovite-rich metapelite crustal source. Most zircon grains from the Songshujiao, Laochang and Kafang granites have high-U concentrations and their SIMS U–Pb ages show age scatter from 81.6 Ma to 88.6 Ma, 80.7 Ma to 86.1 Ma and 82.3 Ma to 87.0 Ma, suggesting formation earlier than the monazite and cassiterite. Monazite SIMS U–Pb ages and Th–Pb ages of three same granite samples are consistent and show yielded 206Pb/238U ages of 83.7  0.6 Ma, 83.7  0.6 Ma, and 83.4  0.6 Ma, and 208Pb/232Th ages of 83.2  0.5 Ma, 83.8  0.4 Ma, and 83.5  0.9 Ma, which are within the range of the SIMS zircon U–Pb ages from these rocks. The data constrain the crystallization of the granites at ca. 83 Ma. In situ U–Pb dating of two cassiterite samples from the cassiterite-sulfide ore in the Songshujiao ore field and Kafang ore field, and two from the cassiterite - oxide þ cassiterite bearing dolomite in the Laochang ore field yielded weighted mean 206Pb/238U ages of 83.5  0.4 Ma (MSWD ¼ 0.6), 83.5  0.4 Ma (MSWD ¼ 0.5), 83.6  0.4 Ma (MSWD ¼ 0.6) and 83.2  0.7 Ma (MSWD ¼ 0.6), respectively. Combined with geological characteristics, the new geochronological data indicate that the formation of the granites and Sn polymetallic deposits are coeval. We correlate the magmatic and metallogenic event with lithospheric thinning and asthenosphere upwelling in continental extension setting in relation to the eastward subduction of the Neo-Tethys beneath the Sanjiang tectonic domain during Late Cretaceous.
Comparison of machine learning models for gully erosion susceptibility mapping
Alireza Arabameri, Wei Chen, Marco Loche, Xia Zhao, Yang Li, Luigi Lombardo, Artemi Cerda, Biswajeet Pradhan, Dieu Tien Bui
2020, (5): 1609-1620. doi: 10.1016/j.gsf.2019.11.009
Gully erosion is a disruptive phenomenon which extensively affects the Iranian territory, especially in the Northern provinces. A number of studies have been recently undertaken to study this process and to predict it over space and ultimately, in a broader national effort, to limit its negative effects on local communities. We focused on the Bastam watershed where 9.3% of its surface is currently affected by gullying. Machine learning algorithms are currently under the magnifying glass across the geomorphological community for their high predictive ability. However, unlike the bivariate statistical models, their structure does not provide intuitive and quantifiable measures of environmental preconditioning factors. To cope with such weakness, we interpret preconditioning causes on the basis of a bivariate approach namely, Index of Entropy. And, we performed the susceptibility mapping procedure by testing three extensions of a decision tree model namely, Alternating Decision Tree (ADTree), Naïve-Bayes tree (NBTree), and Logistic Model Tree (LMT). We dichotomized the gully information over space into gully presence/absence conditions, which we further explored in their calibration and validation stages. Being the presence/absence information and associated factors identical, the resulting differences are only due to the algorithmic structures of the three models we chose. Such differences are not significant in terms of performances; in fact, the three models produce outstanding predictive AUC measures (ADTree ¼ 0.922; NBTree ¼ 0.939; LMT ¼ 0.944). However, the associated mapping results depict very different patterns where only the LMT is associated with reasonable susceptibility patterns. This is a strong indication of what model combines best performance and mapping for any natural hazard – oriented application.
An origin of the along-arc compositional variation in the Izu-Bonin arc system
Kathryn Durkin, Paterno R. Castillo, Susanne M. Straub, Natsue Abe, Yoshihiko Tamura, Quanshu Yan
2020, (5): 1621-1634. doi: 10.1016/j.gsf.2019.12.004
The Izu-Bonin arc system is sediment-poor (~400 m thick with no accretionary prism) and, therefore, the influence of the altered oceanic crust (AOC) is most likely the source of the documented along-arc lava compositional variations, especially in Pb isotopes. Izu-Bonin arc lava geochemistry suggests an influx of subduction component from an Indian-type AOC. However, samples drilled from the western Pacific geochemical reference site at Integrated Ocean Drilling Program Site 1149 implies subduction of a Pacific-type AOC. To solve the apparent discrepancy of slab input versus arc output in this arc system, samples of the AOC were dredged from vertical fault scarps of the subducting Pacific Plate along a transect from 27.5N to 34.5N. Samples range from tholeiitic to mildly alkalic mid-ocean ridge basalts as well as trachybasalts, basaltic trachyandesites, tephrites, and phono-tephrites. Isotope ratios also exhibit a range of values (87Sr/86Sr ¼ 0.70282–0.70673, 143Nd/144Nd ¼ 0.512552–0.513174, 206Pb/204Pb ¼ 18.43–20.00, 207Pb/204Pb ¼ 15.40–15.67, 208Pb/204Pb ¼ 37.75–39.55). Our results suggest that there is a geochemical variation in the AOC that is neither completely due to seawater or hydrothermal alteration, nor to petrogenetic processes. Rather, this variation is the result of the Pacific-Izanagi Ridge system tapping into a heterogeneous, plume-polluted mantle source during the Mid-Cretaceous volcanic event. The observed Pacific-type AOC is not responsible for the Indian-type Pb isotopic signature of Izu-Bonin arc lavas. This leads us to propose an alternative scenario where the Izu-Bonin arc lava Indian-type Pb isotopic signature originates from slab-derived fluids interacting and adsorbing Pb from an Indian-type mantle wedge through zone-refining.
Conditioned duality between supercontinental ‘assembly’ and ‘breakup’ LIPs
Martin B. Klausen
2020, (5): 1635-1649. doi: 10.1016/j.gsf.2019.12.005
A compilation of 178 more precise ages on 10 potential Large Igneous Provinces (LIPs) across southern Africa, is compared to Earth’s supercontinental cycles, where 5 more prominent LIP-events all formed during the assembly of supercontinents, rather than during breakup. This temporal bias is confirmed by a focused review of field relationships, where these syn-assembly LIPs formed behind active continental arcs; whereas, the remaining postassembly – and likely breakup-related – LIPs never share such associations. Exploring the possibility of two radically different LIP-types, only the two younger breakup events (the Karoo LIP and Gannakouriep Suite) produced basalts with more enriched asthenospheric OIB-signatures; whereas, all assembly LIPs produced basalts with stronger lithospheric, as well as more or less primitive asthenospheric, signatures. A counterintuitive observation of Precambrian breakup LIPs outcropping as smaller fragments that are more peripherally located along craton margins, compared to assembly LIPs as well as the Phanerozoic Karoo breakup LIP, is explained by different preservation potentials during subsequent supercontinental cycles. Thus, further accentuating radical differences between (1) breakup LIPs, preferentially intruding along what evolves to become volcanic rifted margins that are more susceptible to deformation within subsequent orogens, and (2) assembly LIPs, typically emplaced along backarc rifts within more protected cratonic interiors. A conditioned duality is proposed, where assembly LIPs are primarily sustained by thermal blanketing (as well as local arc hydration and rifting) below assembling supercontinents and breakup LIPs more typically form above impinging mantle plumes. Such a duality is further related to an overall dynamic Earth model whereby predominantly supercontinent-orientated ocean lithospheric subduction establishes/revitalizes large low shear velocity provinces (LLSVPs) during assembly LIP-activity, and heating of such LLSVPs by the Earth’s core subsequently leads to a derivation of mantle plumes during supercontinental breakup.
Geochemistry and geochronology of mafic rocks from the Spanish Central System: Constraints on the mantle evolution beneath central Spain
D. Orejana, C. Villaseca, M. Kristoffersen
2020, (5): 1651-1667. doi: 10.1016/j.gsf.2020.01.002
Abstract(130) HTML PDF(0)
The Spanish Central System (SCS) contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity: Ordovician tholeiitic metabasites, Variscan calc-alkaline gabbros (Gb1) and microdiorites (Gb2), shoshonitic monzogabbros (Gb3) and alkaline diabases and lamprophyres (Gb4). Not all of these rocks are accurately dated, and several aspects of their genesis are still poorly understood. We present new whole-rock geochemical data (major and trace elements, and Sr–Nd isotopes), U–Pb and Lu–Hf isotopic ratios on magmatic zircons and 40Ar/39Ar amphibole geochronology results in order to establish a precise chronology for the successive events of magmatism in the SCS, and discuss the nature of their mantle sources. Accurate ages have been determined for the Variscan gabbros (305–294 Ma), the microdiorites (299 Ma) and the accompanying felsic porphyries (292 Ma), the shoshonitic monzogabbros (285 Ma), and the alkaline diabases (274 Ma) and monzosyenites (271–264 Ma). According to this information, the Variscan mafic magmatism would be mainly concentrated in the range of 305–294 Ma, with a final manifestation represented by the minor shoshonitic dykes. The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct pulses: diabases and lamprophyres–monzosyenites. Zircon Hf isotopes evidence the involvement of depleted and slightly enriched mantle sources. The bulk of the εHf values are in the broad range of 8 to þ11, indicative of melting both depleted and enriched mantle regions. The high within-sample Hf isotope variation (up to ~11 epsilon units) shown by samples from the Variscan series (gabbros, microdiorites and monzogabbros) could be explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources. Pressure estimates indicate that the Variscan mafic magmas were extracted from the lithosphere. The Nd–Hf isotopic composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny. Deeper (asthenospheric) mantle levels were involved in the generation of the alkaline suite, whose anomalous negative εHf values (moderately decoupled with respect to radiogenic Nd) could be associated with subducted oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the Permian.
East African topography and volcanism explained by a single, migrating plume
Rakib Hassan, Simon E. Williams, Michael Gurnis, Dietmar Müller
2020, (5): 1669-1680. doi: 10.1016/j.gsf.2020.01.003
Anomalous topographic swells and Cenozoic volcanism in east Africa have been associated with mantle plumes. Several models involving one or more fixed plumes beneath the northeastward migrating African plate have been suggested to explain the space-time distribution of magmatism in east Africa. We devise paleogeographically constrained global models of mantle convection and, based on the evolution of flow in the deepest lower mantle, show that the Afar plume migrated southward throughout its lifetime. The models suggest that the mobile Afar plume provides a dynamically consistent explanation for the spatial extent of the southward propagation of the east African rift system (EARS), which is difficult to explain by the northeastward migration of Africa over one or more fixed plumes alone, over the last 45 Myr. We further show that the age-progression of volcanism associated with the southward propagation of EARS is consistent with the apparent surface hotspot motion that results from southward motion of the modelled Afar plume beneath the northeastward migrating African plate. The models suggest that the Afar plume became weaker as it migrated southwards, consistent with trends observed in the geochemical record.
New discriminant-function-based multidimensional discrimination of mid-ocean ridge and oceanic plateau
Surendra P. Verma, Lorena Díaz-Gonz alez
2020, (5): 1681-1693. doi: 10.1016/j.gsf.2020.01.005
From a compilation of geochemical data for the discrimination of the tectonic settings of mid-ocean ridge (MOR; 3730 samples) and oceanic plateau (OP; 3656 samples), we present two new discriminant functions and diagrams obtained from censored multivariate discordant outlier-free datasets. Ten different sets of data (original concentrations as well as isometric log-ratio transformed (ilr) variables; all 10 major (M) elements as well as all 10 major and 6 trace elements MT) were used to evaluate the quality of discrimination from linear discriminant analysis (LDA) and canonical analysis. Two selected multidimensional models ilrM (9 ilr transformed variables of multi-normally distributed 10 major elements) and ilrMT (15 ilr transformed variables of multi-normally distributed combined 10 major and 6 trace elements), considered as the best or most representative (total of 5650 samples for ilrM and 2858 for ilrMT), provided percent success values, respectively, of 80.9% for the MOR and 81.1% for the OP (ilrM) and 88.5% for the MOR and 90.1% for the OP (ilrMT). Both processes (log-ratio transformation and multi-normality) rendered the percent success values similar for both groups (MOR and OP). The respective discriminant functions were successfully used for four tests from known tectonic settings and four application cases (two for ophiolites and two for Precambrian rocks), documenting thus the utility of the new discrimination procedure for the MOR and OP tectonic settings. Furthermore, we showed that our multidimensional procedure is robust against analytical errors or uncertainties, as well as post-emplacement compositional changes caused by element mobility from both low or high temperature alteration. The robustness against the gain or loss of a single element at a time was also documented, from which the ilrMT model was evaluated as more robust than the ilrM model. A new online computer program MOROPdisc was written in Java Framework ZK, which is freely available for use at our web portal http://tlaloc.ier.unam.mx.
Late Mesozoic topographic evolution of western Transbaikalia: Evidence for rapid geodynamic changes from the Mongol–Okhotsk collision to widespread rifting
A.V. Arzhannikova, E.I. Demonterova, M. Jolivet, S.G. Arzhannikov, E.A. Mikheeva, A.V. Ivanov, V.B. Khubanov, L.A. Pavlova
2020, (5): 1695-1709. doi: 10.1016/j.gsf.2019.12.012
The Mesozoic geodynamic evolution of Transbaikalia has been largely controlled by the scissors-like closure of the Mongol–Okhotsk Ocean that separated Siberia from Mongolia–North China continents. Following the oceanic closure, the tectonic evolution of that region was characterized by collisional uplift and subsequent extension that gave rise to the formation of metamorphic core complexes. This complex tectonic setting prevailed simultaneously between 150 Ma and 110 Ma both in Transbaikalia, North Mongolia, and within the North China Craton. Published paleobotanical and paleontological data show that the oldest Mesozoic basins had formed in western Transbaikalia before the estimated age of extension onset. However no precise geochronological age is available for the onset of extension in Transbaikalia. The Tugnuy Basin, as probably the oldest Mesozoic basin in western Transbaikalia, is a key object to date the onset of extension and following changes in tectonic setting. In this study, U–Pb (LA-ICP-MS) dating of detrital zircons from three key Jurassic sediment formations of the Tugnuy Basin are used to identify the potential source areas of the sediments, understand the changes in sediment routing and provide insights on the topographic evolution of western Transbaikalia. Our results show several significant changes in tectonic regime after the closure of the Mongol–Okhotsk Ocean. A wide uplifted plateau formed during the closure of the Mongol–Okhotsk Ocean, determining the Early Jurassic drainage system reaching the Angara- Vitim batholith to the north and shedding sediments to the continental margin to the South. The following collisional event at the end of the Early Jurassic led to the uplift of the collision zone, which partially inverted the drainage system toward the North. A strike-slip displacement induced by the oblique collision initiated some of the early Transbaikalian depressions, such as the Tugnuy Basin at about 168 Ma. A phase of basin inversion, marked by folding and erosion of the Upper Jurassic sediments, could correspond to the short-term collision event that took place during the latest Jurassic–earliest Cretaceous in the eastern Central Asian Orogenic Belt. The following inversion in tectonic regime from compression to extension is consistent with the mid–lower-crustal extension that led to the formation of the numerous metamorphic core complexes throughout northeastern continental Asia during the Early Cretaceous.
Geochemistry, geochronology and Hf isotope of granitoids in the northern Alxa region: Implications for the Late Paleozoic tectonic evolution of the Central Asian Orogenic Belt
Xiaochen Zhao, Chiyang Liu, Jianqiang Wang, Shaohua Zhang, Yuzhao Guan
2020, (5): 1711-1725. doi: 10.1016/j.gsf.2020.01.009
The tectonic setting of the northern Alxa region during the Late Paleozoic is highly controversial. The key to resolve this controversy is to recognize the Late Paleozoic magmatic processes in the northern Alxa. In this paper, we present new zircon U–Pb ages, Hf-isotopic compositions and whole-rock geochemical data of four granitoids along the Zhusileng-Hangwula Tectonic Belt in the northern Alxa region that could provide critical information about the tectonic evolution of this region. The zircon U–Pb data could be grouped as two phases: Late Devonian granite and diorite (ca. 373–360 Ma), and Late Carboniferous granodiorite (ca. 318 Ma). The Late Devonian granites and diorites are metaluminous to slightly peraluminous, with A/CNK and A/NK ratios of 0.90–1.11 and 0.95–2.19, respectively. The Late Devonian diorites are characterized by high MgO, Cr and Ni contents and MgO# values, together with variable εHf(t) values from –1.0 to þ1.3 and old TDM2 ages varied from 1283 Ma to 1426 Ma, indicating the primary magma was potentially derived from magma mixing of depleted mantle with Mesoproterozoic continental crust. Even though the Late Devonian granites yielded most positive and minor negative εHf(t) values between –1.1 to þ5.7 (three grains are negative) with two-stage model ages (TDM2) of 1003–1438 Ma, they display low MgO, Cr and Ni contents and MgO# values, suggesting that they were mainly derived from juvenile crustal materials, mixed with a small amount of ancient crust. The Late Carboniferous granitoids are metaluminous and medium-K calc-alkaline series, with A/CNK and A/NK ratios ranging from 0.88 to 0.95 and 1.75 to 1.90, respectively. These rocks were potentially derived from juvenile crustal materials mixed with depleted mantle, as evidenced by their high εHf(t) values (þ11.6 to þ14.1) and young TDM2 ages (427 Ma to 586 Ma), as well as high Mg# values, and MgO, Ni and Cr contents. Our data, along with available sedimentary evidence and previous researches, indicate that the Late Devonian and Late Carboniferous rocks are arc-related granitoids under the subduction setting. The identification of arc-related granitoids in the northern Alxa region not only reveals the Late Paleozoic magmatic process in response to the subduction of Paleo Asian Ocean, but also provide significant constrains to the tectonic evolution of the Central Asian Orogenic Belt.
Constructing the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western Bainaimiao arc terrane, southeastern Central Asian Orogenic Belt
Min Liu, Shaocong Lai, Da Zhang, Renzhi Zhu, Jiangfeng Qin, Guangqiang Xiong, Haoran Wang
2020, (5): 1727-1742. doi: 10.1016/j.gsf.2020.01.012
Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt (CAOB). The Bainaimiao arc terrane (BAT) is one of the most important Precambrian microcontinents in southeastern CAOB, however, few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton. This study presents an integrated study of geochronology, zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT. The latest Neoproterozoic (ca. 546 Ma) diabases display low SiO2 (46.52–49.24 wt.%) with high MgO (8.23–14.41 wt.%), Cr (66–542 ppm) and Ni (50–129 ppm), consisting with mantle origin. Their highly negative zircon εHf(t) (12.0 to 24.7) and high Fe/Mn ratios (62.1–81.7) further indicate a significantly enriched mantle source. Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root, we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust. The Early Paleozoic (ca. 475–417 Ma) arc intrusions in western BAT can be divided into Periods I and II at approximately 450 Ma. The Period I (>450 Ma) intrusions contain abundant mafic minerals like hornblende and pyroxene, and show positive zircon εHf(t) (þ1.5 to þ10.9). They are predominantly medium-K calc-alkaline with broad correlations of SiO2 versus various major and trace elements, which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7 kbar. We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts. By contrast, the Period II (450 Ma) intrusions are characterized by variable zircon εHf(t) (15.0 to þ11.5) with irregular variations in most major and trace elements, which are more akin to the arc magmas generated in an open system. The general occurrence of elder inherited zircons, along with the relatively high Mg# (>45) of some samples, call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components. In combination with the Early Paleozoic tectonic melanges flanking western BAT, we infer that the compositional transition from Period I to II can be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB. The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT, but also on the associated crustal construction processes before the final arc-continent accretion.
Implications for the lithospheric structure of Cambay rift zone, western India: Inferences from a magnetotelluric study
Nagarjuna Danda, C.K. Rao, Amit Kumar, P. Rama Rao, P.B.V. Subba Rao
2020, (5): 1743-1754. doi: 10.1016/j.gsf.2020.01.014
Broad-band and long-period magnetotelluric (MT) data were acquired along an east-west trending traverse of nearly 200 km across the Kachchh, Cambay rift basins, and Aravalli-Delhi fold belt (ADFB), western India. The regional strike analysis of MT data indicated an approximate N59E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area. The decomposed transverse electric (TE)- and transverse magnetic (TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions. These studies show a thick (~1–5 km) layer of conductive Tertiary–Mesozoic sediments beneath the Kachchh and Cambay rift basins. The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives, depleted mantle lithosphere, and different Precambrian structural units. The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones, sulfide mineralization within polyphase metamorphic rocks, and/or Aravalli-Delhi sediments/metasediments. The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating, volatile (CO2, H2O) enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume–lithosphere interactions. The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India (WCMI) and thus support the hypothesis proposed by Campbell & Griffiths about the plume–lithosphere interactions.
Thermal conductivity of dense hcp iron: Direct measurements using laser heated diamond anvil cell
Pinku Saha, Aritra Mazumder, Goutam Dev Mukherjee
2020, (5): 1755-1761. doi: 10.1016/j.gsf.2019.12.010
Thermal conductivity (k) of iron is measured up to about 134 GPa. The measurements are carried out using the single sided laser heated diamond anvil cell, where the power absorbed by a Fe metal foil at hotspot is calculated using a novel thermodynamical method. Thermal conductivity of fcc (γ)  Fe increases up to a pressure of about 46 GPa. We find thermal conductivity values in the range of 70–80 Wm1K1 (with an uncertainty of 40%), almost constant with pressure, in the hcp (ε) phase of Fe. We attribute the pressure independent k above 46 GPa to the strong electronic correlation effects driven by the electronic topological transition (ETT). We predict a value of thermal conductivity of ε -Fe of about 40  16 Wm1K1 at the outer core of Earth.
Neoarchean and Rhyacian TTG-Sanukitoid suites in the southern S~ao Francisco Paleocontinent, Brazil: Evidence for diachronous change towards modern tectonics
Henrique Bruno, Vitalino Elizeu, Monica Heilbron, Claudio de Morisson Valeriano, Rob Strachan, Mike Fowler, Samuel Bersan, Hugo Moreira, Ivo Dussin, Luiz Guilherme do Eirado Silva, Miguel Tupinamb a, Julio Almeida, Carla Neto, Craig Storey
2020, (5): 1763-1787. doi: 10.1016/j.gsf.2020.01.015
The southern portion of the S~ao Francisco Palaeocontinent in Brazil is denoted by Archean nuclei and Paleoproterozoic magmatic arcs that were amalgamated during Siderian to Orosirian orogenic processes (ca. 2.4–2.1 Ga). New isotopic U–Pb in zircon and Sm–Nd whole rock combined with major and trace element composition analyses constrain the crystallization history of the Neoarchean Piedade block (at ca. 2.6 Ga) and the Paleoproterozoic Mantiqueira Complex (ca. 2.1–1.9 Ga). These therefore display quite different magmatic histories prior to their amalgamation at ca. 2.05 Ga. Sm–Nd and Rb–Sr isotopes imply a mixed mantle-crustal origin for the samples in both units. A complete Palaeoproterozoic orogenic cycle, from subduction to collision and collapse, is recorded in the Piedade Block and the Mantiqueira Complex. Rhyacian to Orosirian subduction processes (ca. 2.2–2.1 Ga) led to the generation of coeval (ca. 2.16 Ga) TTG suites and sanukitoids, followed by late (2.10–2.02 Ga) high-K granitoids that mark the collisional stage. The collisional accretion of the Mantiqueira Complex against the Piedade Block at 2.08–2.04 Ga is also recorded by granulite facies metamorphism in the latter terrane, along the Ponte Nova suture zone. The collisional stage was closely followed by the emplacement of within-plate tholeiites at ca. 2.04 Ga and by alkaline rocks (syenites and enriched basic rocks) at ca. 1.98 Ga, marking the transition to an extensional tectonic regime. The discovery of two episodes of TTG and sanukitoid magmatism, one during the Neoarchean in the Piedade Complex and another during the Rhyacian in the Mantiqueira Complex, indicates that the onset of subduction-related melting of metasomatized mantle was not restricted to Neoarchean times, as generally believed, but persisted much later into the Paleoproterozoic.
Volcano video data characterized and classified using computer vision and machine learning algorithms
Alex J.C. Witsil, Jeffrey B. Johnson
2020, (5): 1789-1803. doi: 10.1016/j.gsf.2020.01.016.
Video cameras are common at volcano observatories, but their utility is often limited during periods of crisis due to the large data volume from continuous acquisition and time requirements for manual analysis. For cameras to serve as effective monitoring tools, video frames must be synthesized into relevant time series signals and further analyzed to classify and characterize observable activity. In this study, we use computer vision and machine learning algorithms to identify periods of volcanic activity and quantify plume rise velocities from video observations. Data were collected at Villarrica Volcano, Chile from two visible band cameras located ~17 km from the vent that recorded at 0.1 and 30 frames per second between February and April 2015. Over these two months, Villarrica exhibited a diverse range of eruptive activity, including a paroxysmal eruption on 3 March. Prior to and after the eruption, activity included nighttime incandescence, dark and light emissions, inactivity, and periods of cloud cover. We quantify the color and spatial extent of plume emissions using a blob detection algorithm, whose outputs are fed into a trained artificial neural network that categorizes the observable activity into five classes. Activity shifts from primarily nighttime incandescence to ash emissions following the 3 March paroxysm, which likely relates to the reemergence of the buried lava lake. Time periods exhibiting plume emissions are further analyzed using a row and column projection algorithm that identifies plume onsets and calculates apparent plume horizontal and vertical rise velocities. Plume onsets are episodic, occurring with an average period of ~50 s and suggests a puffing style of degassing, which is commonly observed at Villarrica. However, the lack of clear acoustic transients in the accompanying infrasound record suggests puffing may be controlled by atmospheric effects rather than a degassing regime at the vent. Methods presented here offer a generalized toolset for volcano monitors to classify and track emission statistics at a variety of volcanoes to better monitor periods of unrest and ultimately forecast major eruptions.
Mapping favorable groundwater potential recharge zones using a GIS-based analytical hierarchical process and probability frequency ratio model: A case study from an agro-urban region of Pakistan
Arfan Arshad, Zhijie Zhang, Wanchang Zhang, Adil Dilawar
2020, (5): 1805-1819. doi: 10.1016/j.gsf.2019.12.013
Abstract(200) HTML PDF(7)
In Punjab (Pakistan), the increasing population and expansion of land use for agriculture have severely exploited the regional groundwater resources. Intensive pumping has resulted in a rapid decline in the level of the water table as well as its quality. Better management practices and artificial recharge are needed for the development of sustainable groundwater resources. This study proposes a methodology to delineate favorable groundwater potential recharge zones (FPRI) by integrating maps of groundwater potential recharge index (PRI) with the DRASTIC-based groundwater vulnerability index (VI). In order to evaluate both indexes, different thematic layers corresponding to each index were overlaid in ArcGIS. In the overlay analysis, the weights (for various thematic layers) and rating values (for sub-classes) were allocated based on a review of published literature. Both were then normalized and modified using the analytical hierarchical process (AHP) and a frequency ratio model respectively. After evaluating PRI and FPRI, these maps were validated using the area under the curve (AUC) method. The PRI map indicates that 53% of the area assessed exists in very low to low recharge zones, 22% in moderate, and 25% in high to excellent potential recharge zones. The VI map indicates that 38% of the area assessed exists in very low to low vulnerability, 33% in moderate, and 29% in high to very high vulnerability zones. The FPRI map shows that the central region of Punjab is moderately-to-highly favorable for recharge due to its low vulnerability and high recharge potential. During the validation process, it was found that the AUC estimated with modified weights and rating values was 79% and 67%, for PRI and VI indexes, respectively. The AUC was less when evaluated using original weights and rating values taken from published literature. Maps of favorable groundwater potential recharge zones are helpful for planning and implementation of wells and hydraulic structures in this region.
Paleoproterozoic juvenile magmatism within the northeastern sector of the S~ao Francisco paleocontinent: Insights from the shoshonitic high Ba–Sr Montezuma granitoids
Samuel Moreira Bersan, Alice Fernanda de Oliveira Costa, Andr e Danderfer, Francisco Rob erio de Abreu, Cristiano Lana, Glaucia Queiroga, Craig Storey, Hugo Moreira
2020, (5): 1821-1840. doi: 10.1016/j.gsf.2020.01.017
Abstract(133) HTML PDF(1)
New, integrated petrographic, mineral chemistry, whole rock geochemical, zircon and titanite U–Pb geochronology, and zircon Hf isotopic data from the Montezuma granitoids, as well as new geochemical results for its host rocks represented by the Corrego Tingui Complex, provides new insights into the late- to post-collisional evolution of the northeastern S~ao Francisco paleocontinent. U–Pb zircon dates from the Montezuma granitoids spread along the Concordia between ca. 2.2 Ga to 1.8 Ga and comprise distinct groups. Group I have crystallization ages between ca. 2.15 Ga and 2.05 Ga and are interpreted as inherited grains. Group II zircon dates vary from 2.04 Ga to 1.9 Ga and corresponds to the crystallization of the Montezuma granitoids, which were constrained at ca. 2.03 Ga by the titanite U–Pb age. Inverse age zoning is common within the ca. 1.8 Ga Group III zircon ages, being related to fluid isotopic re-setting during the Espinhaco rifiting event. Zircon εHf(t) analysis show dominantly positive values for both Group I (4 to þ9) and II (3 to þ8) zircons and TDM2 model ages of 2.7–2.1 Ga and 2.5–1.95 Ga, respectively. Geochemically, the Montezuma granitoids are weakly peraluminous to metaluminous magnesian granitoids, enriched in LILES and LREE, with high to moderate Mg# and depleted in some of the HFSE. Their lithochemical signature, added to the juvenile signature of both inherited and crystallized zircons, allowed its classification as a shoshonitic high Ba–Sr granitoid related to a late- to post-collisional lithosphere delamination followed by asthenospheric upwelling. In this scenario, the partial melting of the lithospheric mantle interacted with the roots of an accreted juvenile intra-oceanic arc, being these hybrid magma interpreted as the source of the Montezuma granitoids. The Corrego Tinguí Complex host rocks are akin to a syn- to late-collisional volcanic arc granitoids originated from the partial melting of ancient crustal rocks. The results presented in this study have revealed the occurrence of juvenile rocks, probably related to an island arc environment, that are exotic in relation to the Paleo- to Neoarchean crust from the S~ao Francisco paleocontinent’s core.
Topography, structural and exhumation history of the Admiralty Mountains region, northern Victoria Land, Antarctica
Maria Laura Balestrieri, Valerio Olivetti, Federico Rossetti, C ecile Gautheron, Silvia Catt o, Massimiliano Zattin
2020, (5): 1841-1858. doi: 10.1016/j.gsf.2020.01.018
Abstract(115) HTML PDF(4)
The Admiralty Mountains region forms the northern termination of the northern Victoria Land, Antarctica. Few quantitative data are available to reconstruct the Cenozoic morpho-tectonic evolution of this sector of the Antarctic plate, where the Admiralty Mountains region forms the northern termination of the western shoulder of the Mesozoic–Cenozoic West Antarctica Rift System. In this study we combine new low-temperature thermochronological data (apatite fission-track and (U-Th-Sm)/He analyses) with structural and topography analysis. The regional pattern of the fission-track ages shows a general tendency to older ages (80–60 Ma) associated with shortened mean track-lengths in the interior, and younger fission-track ages clustering at 38–26 Ma with long mean track-lengths in the coastal region. Differently from other regions of Victoria Land, the younger ages are found as far as 50–70 km inland. Single grain apatite (U-Th-Sm)/He ages cluster at 50–30 Ma with younger ages in the coastal domain. Topography analysis reveals that the Admiralty Mountains has high local relief, with an area close to the coast, 180 km long and 70 km large, having the highest local relief of >2500 m. This coincides with the location of the youngest fission-track ages. The shape of the area with highest local relief matches the shape of a recently detected low velocity zone beneath the northern TAM, indicating that high topography of the Admiralty Mountains region is likely sustained by a mantle thermal anomaly. We used the obtained constraints on the amount of removed crustal section to reconstruct back-eroded profiles and calculate the erosional load in order to test flexural uplift models. We found that our back-eroded profiles are better reproduced by a constant elastic thickness of intermediate values (Te ¼ 20–30 km). This suggests that, beneath the Admiralty Mountains, the elastic properties of the lithosphere are different with respect to other TAM sectors, likely due to a stationary Cenozoic upper mantle thermal anomaly in the region.
An efficient framework for ensemble of natural disaster simulations as a service
Ujjwal KC, Saurabh Garg, James Hilton
2020, (5): 1859-1873. doi: 10.1016/j.gsf.2020.02.002
Calculations of risk from natural disasters may require ensembles of hundreds of thousands of simulations to accurately quantify the complex relationships between the outcome of a disaster and its contributing factors. Such large ensembles cannot typically be run on a single computer due to the limited computational resources available. Cloud Computing offers an attractive alternative, with an almost unlimited capacity for computation, storage, and network bandwidth. However, there are no clear mechanisms that define how to implement these complex natural disaster ensembles on the Cloud with minimal time and resources. As such, this paper proposes a system framework with two phases of cost optimization to run the ensembles as a service over Cloud. The cost is minimized through efficient distribution of the simulations among the cost-efficient instances and intelligent choice of the instances based on pricing models. We validate the proposed framework using real Cloud environment with real wildfire ensemble scenarios under different user requirements. The experimental results give an edge to the proposed system over the bag-of-task type execution on the Clouds with less cost and better flexibility.
Two new species of Mesozoic tree ferns (Osmundaceae: Osmundacaulis) in Eurasia as evidence of long-term geographic isolation
Yeming Cheng, Fengxiang Liu, Xiaonan Yang, Tongxing Sun
2020, (5): 1875-1888. doi: 10.1016/j.gsf.2020.01.019
The fossil trunks and rhizomes of Osmundaceae provide important information about its evolutionary history. Due to limited records of the Mesozoic trunks and rhizomes in the Eurasia of the Northern Hemisphere, our understanding on the fossil diversity of the Osmundaceae is hindered. Two new species of the Osmundaceae trunks, Osmundacaulis asiatica sp. nov. and Osmundacaulis sinica sp. nov. representing the first discovery of the Mesozoic tree fern genus Osmundacaulis in Eurasia, are described from Wudalianchi and Qiqihar, Heilongjiang Province, Northeast China, which enriches the plant diversity of the Osmundaceae in the Eurasia, and provides vital evidence for studying the distribution, radiation and evolution of the genus during the Cretaceous. The fossil records suggest that Osmundacaulis species may have evolved from a common ancestor, which first appeared in the Australian portion of Pangaea, and then spread to ancient northern North America and ancient East Asia. Since then, they developed into different species through their own evolutionary lines. The Chinese species have a special local feature that the outer cortex is thicker than the inner cortex, in contrast with reported Osmundacaulis species having thinner outer cortex and thicker inner cortex. Long-term geographic isolation may have led to the radiation of diverse Osmundacaulis species and the appearance of region-specific features, such as the thick outer cortex and the thin inner cortex of the Chinese species. Among all reported Osmundacaulis species, the two new species found in China, O. nerii from the Jurassic of Australia and the O. lemonii from the Jurassic of the USA, have special groups of mucilage-sacs inside sclerenchyma ring of petiole base. Mucilage sacs probably originated independently among taxonomic groups, representing convergent adaptations to similar habitats, rather than indicating genetic inheritance from a common ancestor.