Hudson, Thomas S.Kendall, J-MichaelPritchard, Matthew E.Blundy, Jonathan D....
13页
查看更多>>摘要：Uturuncu volcano is situated in the Bolivian Andes, directly above the world's largest crustal body of silicic partial melt, the Altiplano-Puna Magma Body (APMB). Uturuncu last erupted 250,000 years ago, yet is seismically active and lies at the centre of a 70 km diameter uplifted region. Here, we analyse seismicity from 2009 to 2012. Our earthquake locations, using a newly developed velocity model, delineate the top and bottom of the APMB, reveal individual faults, and reconcile differences in depth distribution between previous studies. Spatial clustering analysis of these earthquakes reveals the orientations of the faults, which match stress orientations from seismic anisotropy. Earthquake b-values derived from moment magnitudes (1.44 +/- 0.06) differ significantly from those using local magnitude measurements (0.80 +/- 0.03). From these observations and theoretical justification, we suggest that, if possible, moment magnitudes should be used for accurate b-value analysis. We interpret b-values > 1 in terms of fluid-enhanced seismicity. Shallow seismicity local to Uturuncu yields b-values > 1.1 with some temporal variation, suggesting fluid migration along pre-existing faults in a shallow hydrothermal system, likely driven by advection from the APMB. Intriguingly, events deeper than the APMB also yield large b-values (1.4), mapping the ascent into the lower crust of fluids that we infer as originating from a subducting slab. Cumulatively, these results provide a picture of an active magmatic system, where fluids are exchanged across the more ductile APMB, feeding a shallow, fault-controlled hydrothermal system. Such pathways of fluid ascent may influence our understanding of arc volcanism, control future volcanic eruptions and promote the accumulation of shallow hydrothermal ore deposits. (C) 2021 Published by Elsevier B.V.
Szucs, D.Palfy, J.Rioux, M.Trabucho-Alexandre, J. P....
14页
查看更多>>摘要：The end-Triassic mass extinction is one of the big fiveextinction events in Phanerozoic Earth history. It is linked with the emplacement of the Central Atlantic Magmatic Province and a host of interconnected environmental and climatic responses that caused profound deterioration of terrestrial and marine biospheres. Current understanding, however, is hampered by (i) a geographically limited set of localities and data; (ii) incomplete stratigraphic records caused by low relative sea-level in European sections during the Late Triassic and earliest Jurassic; and (iii) major discrepancies in the estimated duration of the latest Triassic Rhaetian that limit spatiotemporal evaluation of climatic and biotic responses locally and globally. Here, we investigate the Late Triassic-Early Jurassic time interval from a stratigraphically well-preserved sedimentary succession deposited in tropical oceanic Panthalassa. We present diverse new data from the lower McCarthy Formation exposed at Grotto Creek (Wrangell Mountains, southern Alaska), including ammonoid, bivalve, hydrozoan, and conodont biostratigraphy; organic carbon isotope (d13Corg) stratigraphy; and CA-ID TIMS zircon U-Pb dates. These data are consistent with a Norian-Rhaetian Boundary (NRB) of similar to 209 Ma, providing new evidence to support a long duration of the Rhaetian. They also constrain the Triassic-Jurassic boundary (TJB) to a similar to 6 m interval in the section. Our TJB delta C-13(org) record from Grotto Creek, in conjunction with previous data, demonstrates consistent features that not only appear correlative on a global scale but also shows local heterogeneities compared to some Tethyan records. Notably, smaller excursions within a large negative carbon isotope excursion [NCIE] known from Tethyan localities are absent in Panthalassan records. This new comparative isotopic record becomes useful for (i) distinguishing regional overprinting of the global signal; (ii) raising questions about the ubiquity of smaller-scale NCIEs across the TJB; and (iii) highlighting the largely unresolved regional vs. global scale of some presumed carbon cycle perturbations. These paleontological and geochemical data establish the Grotto Creek section as an important Upper Triassic to Lower Jurassic succession due to its paleogeographic position and complete marine record. Our record represents the best documentation of the NRB and TJB intervals from Wrangellia, and likely the entire North American Cordillera. (C) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：The stability of frictional sliding affects the spectrum of fault slip, from slow-slip events to earthquakes. In laboratory experiments, the transition from stable sliding to stick-slip is often explained by the ratio of the stiffness of the loading system to a critical value that depends on effective normal stress and other physical properties. However, theoretical considerations indicate other controls on fault stability that have not been validated experimentally. Here, we exploit the dependence of frictional properties on load-point velocity to explore the dynamics of frictional sliding with gradual variations of frictional properties. We use the period-multiplying and chaotic cycles that appear at the transition between stick-slip and stable sliding as a sensitive indicator of fault stability. In addition to the stiffness ratio, we find that the ratio of the parameters that describe the dependence on velocity and state constitutes another control on the stability of faulting and rupture dynamics. Variations of these two non-dimensional parameters among faults may help explain the wide range of rupture styles and recurrence patterns observed in nature. (C) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：Nucleosynthetic Fe isotopic anomalies in meteorites may be used to learn about the early evolution of the solar system and to identify the origin and nature of the material that built the terrestrial planets. Using high-precision iron isotopic data of 23 iron meteorites from nine major chemical groups we show that all iron meteorites define the same dichotomy between non-carbonaceous(NC) and a carbonaceous(CC) meteorites previously observed for other elements. The Fe isotopic anomalies are predominantly produced by variations in Fe-54, where all CC iron meteorites are characterized by an excess in Fe-54 relative to NC iron meteorites. This excess in Fe-54 is accompanied by an excess in Ni-58 observed in the same CC meteorite groups. Together, these overabundances of Fe-54 and Ni-58 are explained by nuclear statistical equilibriumeither in type Ia supernovae or in the Si/S shell of core-collapse supernovae. The Fe isotopic composition of Earth's mantle plots on or close to correlations defined by Fe, Mo, and Ru isotopic anomalies in iron meteorites, indicating that throughout Earth's accretion, the isotopic composition of its building blocks did not drastically change. While Earth's mantle has a similar Fe isotopic composition to CI chondrites, the latter are clearly distinct from Earth's mantle for other elements (e.g., Cr and Ni) whose delivery to Earth coincided with Fe. The fact that CI chondrites exhibit large Cr and Ni isotopic anomalies relative to Earth's mantle, therefore, demonstrates that CI chondrites are unlikely to have contributed significant Fe to Earth and are not its main building blocks. (C) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：East Java is characterised by a complex interaction of volcanic and tectonic processes and it is marked by isolated eruptive centres scattered across the back-arc sedimentary basins. In 2006 a large sediment hosted geothermal system named Lusi, pierced the Kendeng basin in East Java and since then it continues in a relentless eruption of mud breccia. To investigate the spatial and structural relationships between the volcanic arc and the back-arc domains, we perform a local earthquake tomography. The inversion of regional earthquakes recorded by our seismic network (for about two years) shows sharp V-p and V-p/V-s transitions. We observe a marked reduction of P-wave velocities and a high V-p/V-s ratio in the back-arc basins. Our study highlights a clear connection between the plumbing system of the volcanic arc and the northern sedimentary province. We propose a conceptual model suggesting that magmas and hydrothermal fluids may migrate from the middle to the upper crust into the sedimentary basins capitalising on existing thrust faults. Such low angle faults, promoted by the compressional regime of the region, link the magmatic domain to the northern sedimentary provinces. This process may represent the early phase of volcanic arc migration when magma-derived fluids are focused into fractured and permeable geological structures. Our conceptual model would not only help to understand the occurrence of the abundant mantle-derived fluids sampled across the back-arc, but it is also consistent with the occurrence of isolated magmatic and hybrid systems piercing across sedimentary environments in the back-arc of Java. (C) 2021 The Author(s). Published by Elsevier B.V.
查看更多>>摘要：The replacement of magnetite by hematite is commonly observed in various geologic systems. In contrast to the formation of hematite by a solid-state oxidation, numerous experimental results have demonstrated that it can also occur by a redox-independent dissolution-reprecipitation reaction. However, the orientation relationship and the intermediate products between magnetite and hematite remain unknown during the redox-independent replacement. In this work, high-resolution transmission electron microscopy (TEM) was used to investigate porous hematite from the Baishiya skarn deposit, East Kunlun orogenic belt, aiming to build a refined growth model applicable to the replacement of magnetite by hematite in natural hydrothermal systems. Initially, hydrothermal leaching of Fe2+ from magnetite led to the formation of ferrihydrite that transformed to goethite and hematite nanocrystals successively. Oriented attachment of pseudocubic hematite nanoparticles induced by Cl along specific crystallographic directions formed hematite mesocrystals on the exposed dodecahedral facets of magnetite, leading to an orientation relationship between magnetite and adjacent hematite (i.e., (110)(magnetite)parallel to (012)(hematite)), which is different from that of oxidation (i.e., (110)(magnetite)parallel to (110)(hematite)). However, oriented attachment can be imperfect in some instances, and dislocations of adjacent nanoparticles result. The dislocations in the hematite mesocrystals have acted as a closed space to capture the remaining ferrihydrite. When the Si concentration in ferrihydrite was sufficient, the solid-state conversion of the remaining ferrihydrite to hematite was blocked. We propose that repeated dissolution and reprecipitation of hematite mesocrystals (i.e., defective crystals) are required to remove Si, and thereby form defect-free hematite crystals, providing a genetic link between the widespread hematitisation and related multistage fluid infiltration in some of the world's richest deposits (e.g., Olympic Dam and Bayan Obo deposits). This is the first time that Si and Cl have been verified to act as key factors to shape the hematite growth pathway during the ore-forming processes, challenging the `ion-by-ion' growth model that has underpinned our knowledge of mineral solubility, nucleation, and mass transfer from nano- to micron-scales in natural hydrothermal systems. (C) 2021 Elsevier B.V. All rights reserved.
Spencer, C. J.Morrell, T. R.Lu, G. M.Liebmann, J....
9页
查看更多>>摘要：The detrital zircon oxygen isotope record has been used to describe 4.4 billion years of crustal evolution and maturation. Implicit in the application and use of large databases of oxygen isotope ratio (delta O-18) from detrital zircon is that such data are more representative of the history of the continental crust than are exposed rocks. Here we show that the delta O-18 of igneous zircon in preserved rocks varies greatly from the detrital zircon record over time, both in terms of the magnitude of delta O-18 and the secular variation. Starting from 4.0 Ga and lasting throughout the Archean, the detrital zircon record shows consistently higher delta O-18 values than the rock record. In post-Archean times, detrital zircon delta O-18 is conspicuously higher during periods associated with supercontinent amalgamation. Differences between detrital and igneous zircon delta O-18 records imply that the igneous source rocks from which high delta O-18 detrital zircon are derived have been entirely eroded and are not represented in the known geological record. We postulate that the 'missing' record of high delta O-18 magmatic rocks consists of upper crustal sedimentary-derived melts from collisional belts, such as that found in the Himalayan orogen. The observation of higher delta O-18 in Archean detrital zircon further requires a revision of the current paradigm for Archean magmatism. More broadly, recognition of a `missing' component of the upper crustal rock record calls into question the degree to which the detrital zircon record accurately reflects the composition of the bulk continental crust. (C) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：The Gulf of Mexico is an isolated oceanic basin whose nature, structure and age are not fully elucidated, mostly because seafloor spreading isochrons have not been identified in this basin so far. We compiled and processed all publicly available marine magnetic data to produce a new magnetic anomaly map of the Gulf of Mexico. This map reveals a fan-like set of intermediate-wavelength (>100 km) magnetic anomalies related to seafloor spreading. Our magnetic anomaly-based plate reconstructions (1) support a counterclockwise rotation of the Yucatan Block around a pole located NW of Cuba, (2) accommodate the fracture zone trends depicted by the gravity data, and (3) suggest that the Continent-Ocean Boundary lies immediately south of the Houston magnetic anomaly, close to the shoreline, implying that oceanic crust underlies a significant part of the Sigsbee salt province. Our attempt to identify the intermediate wavelength anomalies by comparison with filtered Geomagnetic Polarity Time Scales dates the onset of seafloor spreading before the Tithonian (>150 Ma) and its cessation at the Berriasian (similar to 140 Ma). (C) 2021 Elsevier B.V. All rights reserved.
Sun, MengBezada, Maximiliano J.Cornthwaite, JohnPrieto, German A....
13页
查看更多>>摘要：Both the Caribbean and Nazca plates subduct beneath northwestern South America. The configuration of the two subducted slabs and the nature of any interaction between them has long been a matter of debate. Based on the location of intermediate-depth seismicity and active and extinct volcanism, as well as on seismic imaging, several different tectonic scenarios have been proposed. In this paper, we use teleseismic data recorded by the Colombian National Network and the temporary CARMA array in Venezuela and Colombia to produce a finite-frequency tomography model for the region. Our results show several distinct subduction segments. Through synthetic tests, we show that our results require a zone of overlap between Nazca and Caribbean subduction north of the "Caldas Tear" as has been proposed by previous studies. Additionally, we find that the Bucaramanga Nest occurs within the Caribbean Plate and coincides with bending of the slab in two planes, where both the strike and the dip of the slab change. We infer that elevated stresses are an important factor in producing the very high rates of seismicity in the nest. (C) 2021 Elsevier B.V. All rights reserved.
Yang, JianghaiCawood, Peter A.Condon, Dan J.Liu, Jianzhong...
11页
查看更多>>摘要：Weathering of basalt in tropical environments is sensitive to climate change and considered to play a significant role in regulating climate evolution. Though extensively studied on the modern Earth and hypothesized for the geological past, basalt weathering is difficult to directly track in geological archives. We here focus on the basaltic sediments derived from the Emeishan large igneous province (LIP) in South China and determined their organic carbon content and isotope, mineral and geochemical compositions. Based on the newly obtained high-precision zircon U-Pb age of 251.79 +/- 0.12/0.13/0.36 Ma, we establish temporal variations of weathering intensity for the Emeishan LIP basaltic landscapes in the tropical latitudes through the Permo-Triassic rapid climate warming periods. Analysis of weathering trends indicates a significant decrease in weathering intensity across the Permo-Triassic transition. This decrease in intensity is anomalous considering its coincidence with the climate warming at the Permo-Triassic transition, which tends to elevate chemical weathering rate of basalts and produce sediments with high chemical weathering intensity. Combining these weathering trends with present-day basalt weathering-temperature relationship and reported paleo-temperatures, total erosion rate is quantitatively estimated to increase from 282 +/- 43 t/km(2)/yr in the latest Permian to > 1618 +/- 187 t/km(2)/yr in the Early Triassic. This massive erosion in the Early Triassic is plausibly related to amplified extreme precipitation associated with intensification of the hydrological cycle in the Early Triassic hothouse climate. This study underlines the weathering responses of tropical landscapes to rapid climate warming. (C) 2021 Elsevier B.V. All rights reserved.
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