U-Pb zircon geochronology, geochemistry and Nd isotopic study of Neoproterozoic bimodal volcanic rocks in the Kangdian Rift of South China : Implications for the initial rifting of Rodinia

Both oceanic and continental rifts show regular along-axis segmentation, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systematic along-axis variations in extension and magmatism, making it possible to explore the links between strain, magmatism, and the length scales of faulting and depositional systems during rift development. We summarize Quaternary along-axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segmentation in a “continental” rift sector, the Main Ethiopian Rift (MER) system to the south. We use high-resolution satellite imagery calibrated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing geological reports. From south to north we see a significant and systematic decrease in the lengths of young basin-bounding normal faults (∼50 km long in MER to ≤ 15 km long in northern Afar), in the lengths and widths of rift basins (from 50 to 100 km long × 30 to 80 km wide in MER to ≤ 20 km long × ≤ 5 km wide in northern Afar), and in the relief of the uplifted rift flanks surrounding the basins (from ≥ 1000 m high in MER to ≤ 100 m high in northern Afar). Erosional escarpments along the older, outer rift margins of Afar reveal long, high border fault segments similar to those found in the less evolved MER, suggesting that the Afar Rift was originally bounded by much longer faults. In northern Afar, there is a drastic increase in the volume of Quaternary basalts, with segmentation now dominated by 50- to 80-km-long volcanic ridges, which are similar in size, morphology, and spacing to the second order, nontransform offset segmentation of slow-spreading mid-oceanic ridges. The along-axis changes in rift segmentation are concomitant with decreasing crustal thickness (i.e., cumulative strain), decreasing effective elastic thickness estimates, and increasing magma supply. These temporal and spatial patterns suggest that segmented continental rifts can evolve into segmented oceanic rifts dominated by mantle sourced magmatic processes.

Variations in the along‐axis segmentation of the Afar Rift system

The Ethiopian plateau is made up of several distinct volcanic centres of different ages and magmatic affinities. In the NE, a thick sequence of 30 Ma flood basalts is overlain by the 30 Ma Simien shield volcano. The flood basalts and most of this shield volcano, except for a thin veneer of alkali basalt, are tholeiitic. In the centre of the province, a far thinner sequence of flood basalt is overlain by the 22 Ma Choke and Guguflu shield volcanoes. Like the underlying flood basalts, these shields are composed of alkaline lavas. A third type of magma, which also erupted at 30 Ma, is more magnesian, alkaline and strongly enriched in incompatible trace elements. Eruption of this magma was confined to the NE of the province, a region where the lava flows are steeply tilted as a result of deformation contemporaneous with their emplacement. Younger shields (e.g. Mt Guna, 10·7 Ma) are composed of Si-undersaturated lavas. The three main types of magma have very different major and trace element characteristics ranging from compositions low in incompatible elements in the tholeiites [e.g. 10 ppm La at 7 wt % MgO (=La7), La/Υb = 4.2], moderate in the alkali basalts (La7 = 24, La/Υb = 9.2), and very high in the magnesian alkaline magmas (La7 = 43, La/Υb = 17). Although their Nd and Sr isotope compositions are similar, Pb isotopic compositions vary considerably; 206Pb/204Pb varies in the range of ∼17·9-18·6 in the tholeiites and ∼19·0-19·6 in the 22 Ma shields. A conventional model of melting in a mantle plume, or series of plumes, cannot explain the synchronous eruption of incompatible-element-poor tholeiites and incompatible-element-rich alkali lavas, the large range of Pb isotope compositions and the broad transition from tholeiitic to alkali magmatism during a period of continental rifting. The lithospheric mantle played only a passive role in the volcanism and does not represent a major source of magma. The mantle source of the Ethiopian volcanism can be compared with the broad region of mantle upwelling in the South Pacific that gave rise to the volcanic islands of French Polynesia. Melting in large hotter-than-average parts of the Ethiopian superswell produced the flood basalts; melting in small compositionally distinct regions produced the magmas that fed the shield volcanoes. © Oxford University Press 2004; all rights reserved.

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Flood and shield basalts from Ethiopia: Magmas from the African superswell

Isotopic and trace element signatures of Ethiopian flood basalts: evidence for plume–lithosphere interactions

Two mantle plumes beneath the east African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts.

Afar is a favorable area in which to investigate the interactions between lithosphere and asthenosphere during the opening of a continental rift. The present work focuses on the territory of Djibuti Republic where volcanism has occurred over the past 25 Ma. With the exception of the oldest rhyolites which contain a large crustal-derived component, the Sr, Nd, Pb isotopic compositions of basalts indicate mantle sources without significant crustal contamination. As the rifting process goes on, the sources are evolving from an ancient and isotopically heterogeneous lithospheric subcontinental component (87Sr/86Sr = 0.707 and 206Pb/204Pb = 17.5) to a component having HIMU characteristics (87Sr/86Sr = 0.7035 and 206Pb/204Pb > 19). The influence of the asthenospheric depleted mantle is not established. The present data confirm that a mantle plume is associated with the rifting.

Changes of mantle sources in the course of a rift evolution: The Afar Case

There are two rifts zones in the Republic of Djibuti: the active Asal rift (birthplace of the Ardoukoba basaltic volcano in 1978) and the poorly known Manda-Inakir rift described here. The most recent volcanic event in the Manda-Inakir rift was the formation of the Kammourta basaltic cone, probably in 1928, accompanied by strong seismic activity. This historic eruption and related tectonic features show that the Manda-Inakir rift, like Asal, is presently active. The Kammourta basalt, of transitional alkaline type, belongs to the Manda-Inakir differentiated series, which ranges from basalt to rhyolite. In contrast, volcanic rocks of the Asal rift are entirely transitional tholeiitic basalt. The differences in magmatic affinity and tectonics between these two rift zones reflect the more advanced evolution of rifting in the Asal zone than in Manda-Inakir.

The 1928–1929 eruption of Kammourta volcano — evidence of tectono-magmatic activity in the Manda-Inakir rift and comparison with the Asal Rift, Afar depression, Republic of Djibuti

Datong has for decades been known as China's ‘coal capital’. In 2008 Mayor Geng Yanbo redeveloped Datong's city centre in order to hide the coal stigma and redefine the city's identity based on its cultural heritage as a centre for Buddhist historical sites and ancient architecture. The project was interrupted in 2013 when the mayor was promoted to another city. In parallel, the coal industry was impacted by restructuring, falling prices, and stricter environmental policies. The interrupted project opened a liminal space between demolition and construction. This article studies unfinishedness and ruination in Datong after the construction boom by following the last residents living within the interrupted project. Drawing on fieldwork carried out between 2015 and 2019 that records the discourses and socio-spatial practices of the last remaining residents, stuck between chai (demolition) and qian (relocation), it sheds light on their urban experiences as city dwellers in a landscape of ruins and unfinished architecture. The suspension of urban construction affects both these residents’ representations of the urban space and their ambivalent feelings towards the past, present and future, opening new perspectives for research on the urbanity of booms and busts in Chinese cities. 

J. Audin

Papers

Changes of mantle sources in the course of a rift evolution: The Afar Case

The 1928–1929 eruption of Kammourta volcano — evidence of tectono-magmatic activity in the Manda-Inakir rift and comparison with the Asal Rift, Afar depression, Republic of Djibuti

BETWEEN CHAI AND QIAN : How Unfinishedness and Ruination Have Reshaped Urbanity in China's ‘Coal Capital’ after the Construction Boom