Plume-lithosphere interactions in rifted margin tectonic settings: Inferences from thermo-mechanical modelling
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Mantle plume
Passive margin
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The north China craton (NCC) is one of the oldest cratons in the world; however, the lithosphere of the craton was destructed during Phanerozoic tectonism and then became tectonically and seismically active. Because the lithospheric structure of this complex craton has not been well studied, the mechanism behind the thinning, transformation, and destruction of the lithosphere remains debated. Using an efficient and scalable 3‐D surface wave tomography method, we obtain a high‐resolution regional S wave velocity model that shows the three‐dimensional lithospheric structure of the NCC. In addition, we convert the S wave structure to an estimated thermal structure using accepted relationships between S wave velocity and temperature. The model images a large upper mantle low‐velocity body beneath the eastern NCC, especially beneath the seismically active zone from Tangshan to Xingtai. This body is interpreted to represent hot material or volatiles escaping from the slab edge in the transition zone between the upper and lower mantle. The low‐velocity body is a key piece of evidence in demonstrating thermochemical bottom‐up erosion/transformation of the overlying cratonic lithosphere, thereby leading to destruction of the lithosphere, which may have occurred during the Cenozoic. This erosion mechanism appears to have had less influence in the western NCC (Ordos block); however, our results reveal a ∼130‐km‐thick lithosphere beneath the present cratonic Ordos block, which is thinner than the ∼200 km thickness of the NCC lithosphere during the Paleozoic, as determined from analyses of xenoliths.
Seismic Tomography
Low-velocity zone
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Abstract The nuclei of continents, manifested as cratons, are the most long-lived parts of Earth’s lithosphere. However, ancient cratons in some areas can be substantially destroyed through mechanisms that are not fully understood. We used experimentally calibrated geobarometers to calculate the equilibrium pressures of mafic magmas in the North China craton, which directly constrain the evolving depth of the lithosphere-asthenosphere boundary beneath the craton through time. We show that the lithospheric thickness of the eastern part of the craton decreased from ~200 km to ~35 km in the Early Cretaceous. This intense destruction took place within a short time interval of ~10 m.y., at least locally. Following this destruction, the lithosphere gradually rethickened and stabilized as the upwelling asthenosphere cooled and formed a juvenile lithosphere. We suggest that this catastrophic lithosphere thinning resulted from wholesale lithosphere delamination. As a consequence of this catastrophic loss of thick mantle roots, the eastern part of the North China craton may have undergone rapid crustal rebound and surface uplift, as recorded by the regional unconformities formed between 130 and 120 Ma in the destructed area.
Asthenosphere
Delamination
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Asthenosphere
Receiver function
Lithospheric flexure
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Mantle plume
Hotspot (geology)
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Abstract Recently, the continually increasing availability of seismic data has allowed high‐resolution imaging of lithospheric structure beneath the African cratons. In this study, S‐wave seismic tomography is combined with high resolution satellite gravity data in an integrated approach to investigate the structure of the cratonic lithosphere of Africa. A new model for the Moho depth and data on the crustal density structure is employed along with global dynamic models to calculate residual topography and mantle gravity residuals. Corrections for thermal effects of an initially juvenile mantle are estimated based on S‐wave tomography and mineral physics. Joint inversion of the residuals yields necessary compositional adjustments that allow to recalculate the thermal effects. After several iterations, we obtain a consistent model of upper mantle temperature, thermal and compositional density variations, and Mg# as a measure of depletion, as well as an improved crustal density model. Our results show that thick and cold depleted lithosphere underlies West African, northern to central eastern Congo, and Zimbabwe Cratons. However, for most of these regions, the areal extent of their depleted lithosphere differs from the respective exposed Archean shields. Meanwhile, the lithosphere of Uganda, Tanzania, most of eastern and southern Congo, and the Kaapvaal Craton is thinner, warmer, and shows little or no depletion. Furthermore, the results allow to infer that the lithosphere of the exposed Archean shields of Congo and West African cratons was depleted before the single blocks were merged into their respective cratons.
Seismic Tomography
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Mantle plume
Passive margin
Asthenosphere
Continental Margin
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Abstract We propose that subducting slabs may cause lithospheric removal by directing mantle flow along the craton margin. This process could carve and shape the cratons, leading to conditions that impact the overall (in)stability of the lithosphere. We use three-dimensional geodynamic models to investigate how subduction-driven directed flow interacts with cratonic lithosphere of differing shape, concluding that the margin shape controls both channelization of flow around the craton as well as the potential for destruction. While the simulations show that all craton shapes aid in channelization, the cratons with straight vertical margins are the most resistant to deformation, and the cratons with gradually thickening margins are less resistant to deformation. The dependence on shape could contribute to the progressive removal of cratonic lithosphere along its margin in a runaway process until a more stable vertical margin shape evolves.
Margin (machine learning)
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