Weak and vanishing upper mantle discontinuities generated by large-scale lithospheric delamination in the Longmenshan area, China
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ABSTRACT:
SUBMITTER: He C
PROVIDER: S-EPMC8566558 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:We apply teleseismic P-wave tomography to reconstruct the velocity structure of the Longmenshan area. Our results show possible large-scale delamination beneath the Songpan-Ganzi and Qiangtang terranes, which induced upwelling asthenosphere. Upwelling asthenosphere might have led to lower crust heating, facilitating eastward extrusion of the Songpan Ganzi terrane resulting in localized deformation and uplift along the Longmenshan orogenic belt. We suggest that the eastward extrusion of the Songpan-Ganzi terrane against the rigid lithospheric root of the Sichuan Basin results in stress accumulation and release, leading to large earthquakes in the Longmenshan area.
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Project description:Plate subduction and delamination, two key processes driving plate tectonics, are thought to be controlled by the buoyancy of the lithospheric mantle relative to the underlying asthenosphere. Most mantle delamination models consider a lithospheric density higher than the asthenosphere to ensure negative buoyancy (slab pull). However, mineral physics show that the continental lithospheric mantle density is lighter than the asthenosphere, and that only its pressure-temperature-composition dependence makes it become denser and unstable when sinking adiabatically. Here, we explore the controls on buoyancy using a 2D thermal-diffusive model of plate convergence, considering five chemical compositions and tectonothermal ages, namely Archon (>2.5 Ga), Proton (2.5-1.0 Ga), Tecton (<1.0 Ga), and two oceanic lithospheric plates of 30?Ma and 120?Ma. While the advection of colder rock in oceanic-like plates always results in negative buoyancy, Protons and Tectons exhibit an ability to slowly flip from negative to positive buoyancy at low convergence rates: they first favour the sinking due to advection and then become more buoyant because they are thinner and heat up faster during subduction. In contrast, the lighter density of cratons overprints this effect and hinders delamination or subduction, regardless of the convergence rate. This may explain why Archons are more stable during the Wilson cycle.
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Project description:Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like 18O/16O to lower 'crustal' values. This observation indicates that kimberlites entraining low-18O/16O olivine xenocrysts are modified by assimilation of low-18O/16O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently 'crustal' signatures in mantle-derived magmas.
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Project description:Previous investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role.
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Project description:Ore deposits are loci on Earth where energy and mass flux are greatly enhanced and focussed, acting as magnifying lenses into metal transport, fractionation and concentration mechanisms through the lithosphere. Here we show that the metallogenic architecture of the lithosphere is illuminated by the geochemical signatures of metasomatised mantle rocks and post-subduction magmatic-hydrothermal mineral systems. Our data reveal that anomalously gold and tellurium rich magmatic sulfides in mantle-derived magmas emplaced in the lower crust share a common metallogenic signature with upper crustal porphyry-epithermal ore systems. We propose that a trans-lithospheric continuum exists whereby post-subduction magmas transporting metal-rich sulfide cargoes play a fundamental role in fluxing metals into the crust from metasomatised lithospheric mantle. Therefore, ore deposits are not merely associated with isolated zones where serendipitous happenstance has produced mineralisation. Rather, they are depositional points along the mantle-to-upper crust pathway of magmas and hydrothermal fluids, synthesising the concentrated metallogenic budget available.
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Project description:The source regions of mid-ocean ridge basalts (MORB) are heterogeneous, consisting of chemically and lithologically distinct domains of variable size. Partial melting of such heterogeneous mantle sources gives rise to diverse isotopic compositions of MORB and abyssal peridotites. Variations in radiogenic isotope ratios in MORB are attributed to mixing of melts derived from enriched and depleted mantle components. However, melt mixing alone cannot fully account for the difference between the average 143Nd/144Nd in abyssal peridotites and their spatially associated MORB. We show that the more depleted Nd isotope composition in abyssal peridotites is a natural consequence of melt migration-induced mixing or smearing in the melting column. Sub-kilometer scale enriched mantle components or heterogeneities are significantly damped or homogenized in both the residue and erupted melt during their transit through the melting region. Heterogeneities with larger size and higher incompatible trace element abundance are more resistive to the mixing processes. The size-sensitive mixing depends on a parameter called the enrichment strength, which is the product of the heterogeneity size and the ratio between incompatible trace element abundance in the enriched and depleted mantle sources. Observed Nd-Hf isotope variations in MORB and abyssal peridotites can be reproduced if the enrichment strength is 20 to 60 km. These heterogeneities could be on the kilometer scale and have similar isotope ratios to but less incompatible trace element abundances than recycled oceanic crust.
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