Ocean-Continent Transition (OCT) located between the edge of the continental and unequivocal oceanic crusts is an ideal laboratory to understand one of the most fundamental processes of Plate ...Tectonics, namely the formation mechanism of a new plate boundary, also referred to as lithospheric breakup. However, the location and architecture of the OCT and the processes governing the rupture of continental lithosphere and creation of new oceanic crust remain debated. In this paper, we present newly released high-resolution seismic reflection profiles that image the complete transition from unambiguous continental to oceanic crusts in the mid-northern South China Sea (SCS), accompanied with IODP drill holes and gravity data, with the aim to map the OCT and explore where, when and how lithospheric breakup occurred.
Based on observations and interpretations of seismic reflection data, we define the limits of the SCS OCT. The results show that the OCT basement corresponds to hybrid crust resulting from the complex interaction between crustal thinning along detachment systems and emplacement of new syn-tectonic igneous materials. The lithospheric breakup in the northern SCS and the conjugate margin occurred asymmetrically and was accomplished by core-complex type structures related to an oceanward transition from tectonic to magma-controlled processes during plate separation. Additionally, the observations suggest a sharp along-strike transition from a lower to an upper plate rifted margin setting over a lateral distance of 30 km. The strong variability in the basement architecture and the abrupt flip in detachment polarity imply a transfer zone to explain the segmentation of the margin. Such segmentation may result from inherited pre-rift crustal and/or lithospheric heterogeneities. Notably, the segmentation did not control breakup and subsequent oceanic accretion.
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•Ocean-continent transition in northern South China Sea presents along-strike variation from lower to upper plate setting.•The breakup was accomplished by core-complex type structures.•The variation of the architecture and detachment polarity in the ocean-continent transition suggests the marginal segmentation.
When Hiram Bingham, a historian from Yale University, first saw Machu Picchu in 1911, it was a ruin obscured by overgrowth whose terraces were farmed a by few families. A century later, Machu Picchu ...is a UNESCO world heritage site visited by more than a million tourists annually. This remarkable transformation began with the photographs that accompanied Bingham’s article published in National Geographic magazine, which depicted Machu Picchu as a lost city discovered. Focusing on the practices, technologies, and materializations of Bingham’s three expeditions to Peru (1911, 1912, 1914–1915), this book makes a convincing case that visualization, particularly through the camera, played a decisive role in positioning Machu Picchu as both a scientific discovery and a Peruvian heritage site. Amy Cox Hall argues that while Bingham’s expeditions relied on the labor, knowledge, and support of Peruvian elites, intellectuals, and peasants, the practice of scientific witnessing, and photography specifically, converted Machu Picchu into a cultural artifact fashioned from a distinct way of seeing. Drawing on science and technology studies, she situates letter writing, artifact collecting, and photography as important expeditionary practices that helped shape the way we understand Machu Picchu today. Cox Hall also demonstrates that the photographic evidence was unstable, and, as images circulated worldwide, the “lost city" took on different meanings, especially in Peru, which came to view the site as one of national patrimony in need of protection from expeditions such as Bingham’s.
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•A decaying syn-rift subsidence is observed in the Baiyun Rift in the Eocene.•The average subsidence rate is higher in the western than that in the eastern Baiyun Rift during the Late ...Eocene.•An accelerated post-rift subsidence occurred in eastern Baiyun Rift during the middle Miocene.•This syn- and post-rift subsidence pattern could be induced by the detachment faulting.
How detachment fault controls the basin subsidence at hyper-extended passive margin remains mysterious. Based on a detailed geological interpretation and backstripping on 29 high-resolution multi-channel seismic profiles, we calculate the basin-scale tectonic subsidence across the Baiyun Rift to study the relationship between tectonic subsidence and detachment faulting. Our results confirm that detachment faulting is the primary factor controlling the syn-rift subsidence. A decaying subsidence rate, or deficit subsidence is observed in the eastern Baiyun Rift from the early to late Eocene, which is in marked contrast to the intensive brittle stretching. We attribute this to the doming of the detachment fault footwall involving the syn-rift magmatism during its hyper-extension process. While in the western Baiyun Rift the average subsidence rate is over 45 m Myr−1 higher than that in the eastern as the response of flexural loading in the late Eocene. During the post-rift stage, the subsidence rate shows a similar oceanward increasing trend, and an anomalous rapid subsidence event during the middle Miocene in the eastern Baiyun Rift, which can be ascribed to the magma-assisted weakened rheological condition of the lithosphere deriving from the syn-rift detachment faulting. We propose that this relationship between subsidence pattern and detachment faulting has global applicability for other hyper-extended rift systems developing detachments and magmatic-related dome structures.
Iron (Fe) isotopic compositions of seafloor basalts can provide important insights into high-temperature processes in the upper mantle. However, low-temperature seawater alteration (i.e., seafloor ...weathering) may significantly affect the primary Fe isotopic compositions. In this paper, we report whole-rock Fe isotopes, major and trace elements, and SrNd isotopes, along with in situ elemental maps for slightly altered Site U1434 basalts collected on IODP Expedition 349 from the South China Sea. We use these data to investigate the influence of low-temperature seawater alteration on the Fe isotopic compositions of seafloor basalts. The basalts have δ57Fe values of +0.12‰ to +0.27‰. No correlation between δ57Fe values versus Sr or Nd isotopes that are source-sensitive and no correlation between δ57Fe values and La/Yb ratios that are melting-sensitive indicate the δ57Fe values have not been affected by mantle source heterogeneity and partial melting. The negative correlations of MgO versus CaO and Al2O3 contents suggest limited clinopyroxene and plagioclase fractionation during magmatic evolution. Possible influence of olivine fractionation on the Fe isotopic variations can further be ruled out by a positive correlation between MgO and SiO2 contents and a weak negative correlation between δ57Fe values and SiO2 contents because olivine fractionation can form basalts with increasing SiO2 and δ57Fe with decreasing MgO. The strong correlations between δ57Fe values and MgO, Al2O3, and TiO2 contents indicate the Fe isotopic variations were caused by low-temperature seawater alteration. This is also evident from in situ elemental mapping of representative Site U1434 basalt sample, which shows that low-temperature alteration was characterized by decreases in MgO and SiO2 contents, and increases in Al2O3 and TiO2 contents. We propose a two-stage alteration model to explain the Fe isotopic variations of the Site U1434 basalts. The alteration appeared to occur under oxidizing conditions: Fe2+-rich external fluid likely derived from the progressive alteration inside the oceanic crust was oxidized at the surface of the oceanic crust, which caused the alteration, Fe isotopic fractionation, and increase in δ57Fe values of the seafloor basalts. This study highlights how low-temperature seawater alteration under oxidizing conditions can modify the Fe isotopic compositions of slightly altered oceanic crust, with critical implications for understanding the Fe cycle in the oceans.
•Oceanic crustal basalts have variable δ57Fe values under seafloor weathering.•The two-stage alteration model explains elevating δ57Fe values of seafloor basalts.•Seafloor weathering can form a Fe reservoir with lighter Fe isotopes.
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•Normal faults developed in basins/sags with large throws (≥0.5 km) and massifs mainly with small offsets (<0.5 km).•The most intense Cenozoic magmatism is 5.5–2.6 Ma in the NW South ...China Sea.•Upper crustal faulting, magmatic heating, mantle flow shearing and thick sediment loading caused differential extension.
This study investigates the crustal structure and Cenozoic magmatism in the northwestern South China Sea (SCS), based on two long-cable multi-channel seismic reflection profiles, together with gravity and magnetic data, and adjacent wide-angle refraction profiles. Basins/sags are bounded by large listric-normal faults (fault throws ≥ 0.5 km) and massifs are cut off by normal faults with small offsets (fault throws < 0.5 km) in the northwestern SCS. These structures are penetrated by magmatic edifices showing positive gravity and magnetic anomalies. syn-Rift magmatic intrusions/extrusions were intense in the basins/sags and continent-ocean transition zone while post-rift magmatism was widespread from basins/sags to massifs with the most intense stage occurring from 5.5 to 2.6 Ma. Based on previous geophysical and geochemical results, we suggest that syn-rift mantle upwelling from partial melting initiated seafloor spreading magmatic activities, whereas plume-related mantle upwelling contributed to the magmatism during and after seafloor spreading in the northwestern SCS. Stretching factors show that the upper and lower crusts have experienced differential extension from basins/sags to massifs. The non-uniform crustal extension resulted from upper crustal faulting and lower crustal flow. Particularly, the lower crustal flow was probably linked with the combined action of magmatic heating, mantle flow shearing stresses, and sediment loading, resulting in crustal boudinage and reestablishment of an equilibrium state over long distances.
Global monsoon and ocean drilling Wang, Pinxian; Tada, Ryuji; Clemens, Steven
Scientific drilling (Hokkaido, Japan),
10/2018, Letnik:
24
Journal Article
Recenzirano
Odprti dostop
The IODP-PAGES Workshop on Global Monsoon in Long-term Records was held on
7–9 September 2017, in Shanghai, China. Forty-eight scientists from 12
countries exchanged scientific findings from the ...seven recent IODP
monsoon-related expeditions (see Table 1), discussed future research
directions, and strongly recommended that monsoon system behavior be included
in a future IODP initial science plan because it is one of the most active
factors in the global climate system and crucially influences the global
hydrological cycle.
Uncovering a wealth of neglected archival information, this book examines both visual and textual material from the mid-nineteenth century Franklin Search Expeditions to the Arctic, painstakingly ...tracing their influence on popular imagination. Its surprising findings present a compelling challenge to the still-dominant 'man-versus-nature' trope.
The arrival of Spaniards in 1769 served as a defining moment for California's future. They described the First Peoples and their cultures and provided a window into the evolution of California's ...Camino Real. In an effort to establish the Camino Real de California as a UNESCO World Heritage Site, Joseph P. Sánchez explores the rich history of the path running from San Diego to San Francisco in this significant study. While records capture the stories and legends of the Camino Real there is little information on the exact ground route. Sánchez utilizes historical and archaeological literature and the documentation from Spanish and Mexican archives to begin the much-needed process of authentication of this braided corridor to further establish the Camino Real de California's integrity and valuable history, which is shared with Spain, Mexico, and Native American tribes. Their story is part of the patrimony of the Camino Real de California, which ought to be authenticated, preserved, and protected for future generations to enjoy.
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•Continental crust hosts highly titled fault blocks, oceanic crust hosts highly reflective basement.•Crustal thickness decreases from ∼17 to ∼8 km, with slightly local thickening ...beneath Outer Margin High.•Stretching factor shows westward decrease, indicating the progressive westward rifting.•A rather narrow COT (∼18 km) confirms a rapid transition from continental to oceanic crust.•Syn-rift magmatic additions prompt continental breakup and seafloor spreading.
To improve constraints on breakup processes that eventually lead to seafloor spreading and the subsequent formation of new oceanic crust in the northern margin of the South China Sea (SCS), we collect a 172-km-long deep-penetration wide-angle seismic data OBS2018-L5. In the continental domain, the results present a gradually thinning crust with thickness ranging from ∼17 km to ∼8 km, with a slight thickening crust at the Outer Margin High. The attenuated continental crust is fairly rough and highly offset by distinctly different scale faults system. In the oceanic domain, the top of the basement is highly reflective with weak faulting and relatively constant crust thickness ranging from 5 km to 8 km (average 6 km). The continent-ocean boundary is characterized by an abrupt change in crustal thickness accompanied by a specific boundary between mostly positive gravity anomalies in the oceanic basin and grossly negative anomalies in the thinned continental crust, and between low amplitude, out-of-shape magnetic anomalies in the thinned continental domain and high amplitude, linear anomalies in the oceanic domain. A 0–5 km thick high-velocity layer (7.0–7.5 km/s) is present in the distal margin, which might represent mafic magma underplating. We also identify other magmatic features such as distinct buried volcanic edifices overprinting the original velocity structure. We propose that three mechanisms control the extension and breakup of the SCS: (1) the rapid transition from continental to oceanic crust, (2) elevated potential mantle temperature, and (3) the ductile lower crustal necking. Their interactions lead to the intense thinning of the continental lithosphere, decompression melting of the enriched asthenosphere followed by fast upwelling of mafic melt, relatively strong volcanic activities during the final stage of syn-rift and breakup, and finally generating a margin with intermediately rich magma, referred to as in intermediate-type margin.
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