Alkali-carbonate fluids in the lithospheric mantle
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[1] Although presence of weak layers due to hydration and/or metasomatism in the lithospheric mantle of cratons has been detected by both geophysical and geochemical studies, its influence on craton evolution remains elusive. Using a 2‒D thermomechanical viscoelastoplastic numerical model, we studied the craton extension of a heterogeneous lithospheric mantle with a rheologically weak layer. Our results demonstrate that the effect of the weak mantle layer is twofold: (1) enhances deformation of the overlying lithosphere and (2) inhibits deformation of the underlying lithospheric mantle. Depending on the weak‒layer depth, the Moho temperature and extension rate, three extension patterns are found (1) localized mantle necking with exposed weak layer, (2) widespread mantle necking with exposed weak layer, and (3) widespread mantle necking without exposed weak layer. The presence of the weak mantle layer reduces long‒term acting boundary forces required to sustain extensional deformation of the lithosphere.
Necking
Metasomatism
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Collision zone
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A method is proposed for calculating the normal compaction curve of rocks for the deep parts of the lithosphere. It is based on the analysis of petrophysical characteristics and interpretation of gravitational anomalies. The main regularities of density changes in the lithosphere of Northern Eurasia are investigated. It has been shown that the density section of the region’s lithosphere is characterized by alternation of paragenetically connected decompacted-compacted (relative to normal compaction) rock complexes at all levels of the lithosphere from the surface layers of bottom sediments to the mantle. These zones of decompacted-compacted rocks complexes have a global distribution.
Petrophysics
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The lithosphere may be defined as the cool, strong, rocky layer of the Earth that is supported by the denser, weaker, and relatively hotter underlying magma. The upper few kilometers of the lithosphere are traversed by many faults and fractures, but below this surface level it is sufficiently homogeneous to support great superimposed geological loads.
Lithospheric flexure
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The heights of hotspot volcanoes are modeled by assuming isostatic equilibrium between the magma column and the adjacent lithosphere. The depth to the level of compensation is assumed to be related to the thickness of the lithosphere after it has been reheated and thinned by a hotspot. There is a square‐root relationship between volcano height and lithospheric age. Relationships between volcano height and lithospheric thickness and between reheated thickness and age of the lithosphere can be constrained within the limits imposed by uncertainties in lithospheric and magma densities. If the reheated thickness determined from the isostatic model can be compared with lithospheric thickness determined from thermal models of the lithosphere, then a relationship between the thickness of the lithosphere before and after reheating can be derived. Combining the upper limit on the reheated thickness/age relationship with a theoretical expression, derived by S. T. Crough, which relates swell height to lithospheric age, reset thickness, and physical constants, yields a swell‐height/age relationship that is in good agreement with empirical swell‐height data. This agreement supports the assumption that the lithospheric thickness defined isostatically by volcano heights is closely related to the thermal thickness of the lithosphere, a result that, in turn, supports the thermal origin of hotspot swells.
Lithospheric flexure
Hotspot (geology)
Swell
Asthenosphere
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Abstract Density anomalies beneath the lithosphere are expected to generate dynamic topography at the Earth's surface due to the induced mantle flow stresses which scale linearly with density anomalies, while the viscosity of the upper mantle is expected to control uplift rates. However, limited attention has been given to the role of the lithosphere. Here we present results from analogue modeling of the interactions between a density anomaly rising in the mantle and the lithosphere in a Newtonian system. We find that, for instabilities with wavelengths of the same order of magnitude as lithosphere thickness, the uplift rate and the geometry of the surface bulge are inversely correlated to the lithosphere thickness. We also show that a layered lithosphere may modulate the topographic signal. With respect to previous approaches our models represent a novel attempt to unravel the way normal stresses generated by mantle flow are transmitted through a rheologically stratified lithosphere and the resulting topographic signal.
Lithospheric flexure
Ocean surface topography
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We estimate the elastic thickness of a continental lithosphere by using two approaches that combine the Vening Meinesz-Moritz (VMM) regional isostatic principle with isostatic flexure models formulated based on solving flexural differential equations for a thin elastic shell with and without considering a shell curvature.To model the response of the lithosphere on a load more realistically, we also consider lithospheric density heterogeneities.Resulting expressions describe a functional relation between gravity field quantities and mechanical properties of the lithosphere, namely Young's modulus and Poisson's ratio that are computed from seismic velocity models in prior of estimating the lithospheric elastic thickness.Our numerical study in central Eurasia reveals that both results have a similar spatial pattern, despite exhibiting also some large localized differences due to disregarding the shell curvature.Results show that cratonic formations of North China and Tarim Cratons, Turan Platform as well as parts of Siberian Craton are characterized by the maximum lithospheric elastic thickness.Indian Craton, on the other hand, is not clearly manifested.Minima of the elastic thickness typically correspond with locations of active continental tectonic margins, major orogens (Tibet, Himalaya and parts of Central Asian Orogenic Belt) and an extended continental crust.These findings generally support the hypothesis that tectonically active zones and orogens have a relatively small lithospheric strength, resulting in a significant respond of the lithosphere on various tectonic loads, compared to a large lithospheric strength of cratonic formations.
Lithospheric flexure
Obduction
Isostasy
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Asthenosphere
Lithospheric flexure
Low-velocity zone
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