Low velocity crustal flow and crust–mantle coupling mechanism in Yunnan, SE Tibet, revealed by 3D S-wave velocity and azimuthal anisotropy
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Low-velocity zone
Asthenosphere
Seismic anisotropy
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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.
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Delamination
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The article presents the data calculated from four different viscosity structures V1, V2 [1], SH08 [2], and GHW13 [3], as well as two tomography models S40RTS [4] and SAW642AN [5], using the joint modeling of lithosphere and mantle dynamics technique [3, 6-9]. Besides, the data contain the information on the viscosity variations of the lithosphere, asthenosphere, transition zone, and D″ layer based on the viscosity structure SH08.
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Archean cratons have thick, cold lithosphere that is remarkably stable, thanks to its compositional buoyancy and mechanical strength. Despite this stability, cratonic lithosphere can, sometimes, be modified and eroded, following the impact of a mantle plume, episodes of subduction and continental collision, or stretching and rifting. Although the chemical modification and removal of the Archean lithospheric material are permanent, there is intriguing evidence for re-growth in cratonic lithosphere’s thickness in some locations. In order to understand the enigmatic lithospheric evolution of cratons and continental blocks adjacent to them, we need the knowledge of the thermo-chemical structure of the lithosphere and of the dynamics of the lithosphere-asthenosphere interaction.Seismic surface waves yield abundant evidence on the thermal structure and thickness of the lithosphere and on the temperature of the underlying upper mantle. Tomographic maps resolve in fine regional detail the boundaries between high-velocity (cold) cratons and lower-velocity (warm) neighbouring blocks. The radial structure and thickness of the lithosphere, however, are not resolved by tomographic models quite as well, due to their non-uniqueness. As a result, seismic-velocity profiles from tomographic models are normally incompatible with plausible geotherms. How, then, can we determine the structure and thickness of the lithosphere?Recently developed methods for computational-petrology-powered inversion (e.g., Fullea et al. 2021) relate seismic, topography, heat-flow and other data directly to temperature and composition of the lithosphere and underlying asthenosphere. The misfit valleys in the surface-wave-dominated parameter space are still broad, and it is essential to have accurate measurements and low data-synthetic misfits. Here, we achieve remarkably low misfits of ~0.1% of the surface-wave phase-velocity values by precise tuning of the petrological inversion, its parameterisation and regularisation. The data are fit closely by models with depleted harzburgite mantle compositions within the lithosphere of cratons. The inversions tightly constrain the thickness of cratonic lithosphere, which we find to vary in the ~150-300 km range over different cratons. The plume-lithosphere interactions and the associated surface uplift and volcanism are controlled, to a large extent, by the lithospheric thickness  (e.g., Civiero et al. 2022), which, in turn, evolves with time, influenced by the processes. High-resolution seismic imaging and the petrological inversion of the resulting data yield exciting new discoveries on the evolution of continental lithosphere and its interactions with the underlying mantle.ReferencesCiviero, C., Lebedev, S., Celli, N. L., 2022. A complex mantle plume head below East Africa-Arabia shaped by the lithosphere-asthenosphere boundary topography. Geochemistry, Geophysics, Geosystems, 23, e2022GC010610.Fullea, J., Lebedev, S., Martinec, Z., Celli, N.L., 2021. WINTERC-G: mapping the upper mantle thermochemical heterogeneity from coupled geophysical–petrological inversion of seismic waveforms, heat flow, surface elevation and gravity satellite data. Geophysical Journal International, 226(1), 146-191.
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Low-velocity zone
Seismic Tomography
Lithospheric flexure
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Abstract The Iranian plateau is a natural laboratory for deciphering the lithospheric deformation and deep dynamics in response to the Neo‐Tethyan subduction and subsequent Arabia‐Eurasia continental collision. Here we used S‐wave receiver function data from a dense seismic array to construct the structural image of the lithosphere‐asthenosphere system across the northeastern to eastern Iranian plateau. The lithosphere‐asthenosphere boundary (LAB) is consistently imaged, being relatively flat at 80–90 km depth in eastern Iran, but deepening northward to ∼120 km depth beneath the Kopeh Dagh‐Turan platform boundary with an overall “root‐like” shape in northeastern Iran. A shallower (<45 km) and a deeper Moho (∼50–55 km) are also observed beneath eastern and northeastern Iran, respectively, suggesting similar crustal deformation to mantle lithosphere but different patterns laterally. Moreover, a strong positive velocity discontinuity (PVD) is detected at ∼170 km depth beneath eastern Iran, possibly representing the base of an asthenospheric low‐velocity layer (LVL). Waveform modeling reveals large velocity contrasts of ∼4%–6% over a depth range of less than 30 and 20 km, respectively, across the LAB and the asthenospheric PVD. Combining our new results with geological and petrological constraints, we deduced that the asthenospheric LVL contains a small amount of melt, with sharp boundaries separating from the nearly melt‐free lithosphere above (LAB) and deeper mantle below (PVD). The observed structural variations of the lithosphere‐asthenosphere system across the study region may result from the complex lithospheric deformation in response to the Arabia‐Eurasia collision and effects of multiple tectono‐thermal events associated with the earlier Neo‐Tethyan subduction.
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Collision zone
Low-velocity zone
Discontinuity (linguistics)
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Author(s): Stubailo, Igor | Advisor(s): Davis, Paul M | Abstract: We use data from seismic networks with unprecedented dense coverage to study the Earth's structure under Mexico. First, we develop a three-dimensional (3-D) model of shear-wave velocity and anisotropy for the Mexico subduction zone using fundamental mode Rayleigh wave phase velocity dispersion measurements. The 3-D nature of our surface-wave-based results allows for better understanding of the interaction between the subducting slab, mantle lithosphere, and asthenosphere in the top 200 km. Our phase velocity maps reveal lateral variations at all periods consistent with the presence of flat and steep subduction. We also find that the data are consistent with two layers of anisotropy beneath Mexico: a crustal layer and a deeper layer that includes the lithosphere and asthenosphere, with the fast direction interpreted as aligned with the toroidal mantle flow around the slab edges. Our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure. Our anisotropy results favor a tear, which is also consistent with the geometry of the volcanic belt.Next, we conduct a shear wave splitting analysis that results in delay times of 1-2 s and the fast direction that coincides with the absolute plate motion for the Mesoamerican Seismic Experiment (MASE) stations as well as stations east of the MASE array. The significant difference of the anisotropy in the upper 200 km, as detected by the surface wave analysis, and the average anisotropy between the CMB and the surface, as resolved by the shear wave splitting, implies that the shear wave splitting results are dominated by a structure deeper than 200 km. Since the time delays are significantly longer for the shear wave splitting results, the deeper structure is either much larger than 200 km, or has stronger anisotropy than the top 200 km, or a combination of both. At the same time, several relatively subtle features in the shear wave splitting results reveal potential influences of the shallow structure and its deeper extensions. This includes a small change in the fast direction around the southern edge of the Trans-Mexican Volcanic Belt (TMVB), which is located above the transition from the flat to steep subduction, as well as a different pattern of fast directions west of the MASE array, the region on top of two smaller subducting slabs.Finally, we determine phase velocities of higher modes of Rayleigh waves, in order to constrain the depth of the anisotropy revealed by the shear wave splitting. Our analysis shows that the phase velocities for a number of overtones and periods are fastest in the direction predicted by shear wave splitting, suggesting that they are affected by the same deeper structure. Remarkably, the results for different directions are consistent with the presence of azimuthal anisotropy. Inspection of obtained phase velocities together with the sensitivity kernels tentatively indicates that a layer at the 200-400 km depth is a likely candidate for the source of the anisotropy. We find that such a layer can reproduce the observed shear wave splitting delays for reasonable values of anisotropy. The 200-400 km depth likely corresponds to the bottom of the asthenosphere, and it may be affected by the plate motion, explaining why the fast shear wave splitting direction is aligned with the plate motion. This tentative estimate of the anisotropy depth is consistent with findings in Northern Australia.
Asthenosphere
Seismic anisotropy
Slab
Shear wave splitting
Low-velocity zone
Rayleigh Wave
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Abstract Earth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of S p waves to constrain the lithosphere‐asthenosphere velocity gradient at long‐lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2–3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere‐asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere‐asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.
Asthenosphere
Low-velocity zone
Lithospheric flexure
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Asthenosphere
Low-velocity zone
Lithospheric flexure
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