Small-Scale Heterogeneities of the Upper Mantle

1997 ◽  
pp. 215-223 ◽  
Author(s):  
F. Wenzel ◽  
K. Fuchs ◽  
M. Tittgemeyer ◽  
T. Ryberg
Keyword(s):  
Upper Mantle ◽  
Small Scale

African Hot Spot Volcanism: Small-Scale Convection in the Upper Mantle Beneath Cratons

Science ◽  
2000 ◽  
Vol 290 (5494) ◽  
pp. 1137-1140 ◽  
Author(s):  
S. D. King
Keyword(s):  
Upper Mantle ◽  
Hot Spot ◽  
Small Scale

The consequences of Canadian Cordillera thermal regime in recent tectonics and elevation: a reviewThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.Geological Survey of Canada Contribution 20090195.

2010 ◽  
Vol 47 (5) ◽  
pp. 621-632 ◽  
Author(s):  
R. D. Hyndman
Keyword(s):  
Thermal Regime ◽  
Upper Mantle ◽  
High Surface ◽  
Plate Boundary ◽  
High Elevation ◽  
Mantle Xenoliths ◽  
Tectonic Activity ◽  
Small Scale ◽  
Canadian Cordillera ◽  
Crust And Upper Mantle

The crust and upper mantle thermal regime of the Canadian Cordillera and its tectonic consequences were an important part of the Cordillera Lithoprobe program and related studies. This article provides a review, first of the thermal constraints, and then of consequences in high surface elevation and current tectonics. Cordillera and adjacent craton temperatures are well constrained by geothermal heat flow, mantle tomography velocities, upper mantle xenoliths, and the effective elastic thickness, Te. Cordillera temperatures are very high and laterally uniform, explained by small scale convection beneath a thin lithosphere, 800–900 °C at the Moho, contrasted to 400–500 °C for the craton. The high temperatures provide an explanation for why the Cordillera has high elevation in spite of a generally thin crust, ∼33 km, in contrast to low elevation and thicker crust, 40–45 km, for the craton. The Cordillera is supported ∼1600 m by lithosphere thermal expansion. In the Cordillera only the upper crust has significant strength; Te ∼ 15 km, in contrast to over 60 km for the craton. The Cordillera is tectonically active because the lithosphere is sufficiently weak to be deformed by plate boundary and gravitational forces; the craton is too strong. The Canadian Cordillera results have led to new understandings of processes in backarcs globally. High backarc temperatures and weak lithospheres explain the tectonic activity over long geological times of mobile belts that make up about 20% of continents. They also have led to a new understanding of collision orogenic heat in terms of incorporation of already hot backarcs.

scholarly journals Noble gas systematics in new popping rocks from the Mid-Atlantic Ridge (14°N): Evidence for small-scale upper mantle heterogeneities

2019 ◽  
Vol 519 ◽  
pp. 70-82 ◽  
Author(s):  
Sandrine Péron ◽  
Manuel A. Moreira ◽  
Mark D. Kurz ◽  
Joshua Curtice ◽  
Jerzy S. Blusztajn ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Noble Gas ◽  
Small Scale ◽  
Mantle Heterogeneities ◽  
Mid Atlantic Ridge

Thermal state of the upper mantle and the origin of the Cambrian-Ordovician ophiolite pulse: Constraints from ultramafic dikes of the Hayachine-Miyamori ophiolite

2020 ◽  
Vol 105 (12) ◽  
pp. 1778-1801
Author(s):  
Takafumi Kimura ◽  
Kazuhito Ozawa ◽  
Takeshi Kuritani ◽  
Tsuyoshi Iizuka ◽  
Mitsuhiro Nakagawa
Keyword(s):  
Mantle Source ◽  
Upper Mantle ◽  
Mantle Wedge ◽  
Thermal State ◽  
Potential Temperature ◽  
Geochemical Data ◽  
Small Scale ◽  
Stability Field ◽  
Depleted Mantle ◽  
Slab Breakoff

Abstract Ophiolite pulses, which are periods of enhanced ophiolite generation and emplacement, are thought to have a relevance to highly active superplumes (superplume model). However, the Cambrian-Ordovician pulse has two critical geological features that cannot be explained by such a superplume model: predominance of subduction-related ophiolites and scarcity of plume-related magma activities. We addressed this issue by estimating the mechanism and condition of magma generation, including mantle potential temperature (MPT), from a ~500 Ma subduction-related ophiolite, the Hayachine-Miyamori ophiolite. We developed a novel method to overcome difficulties in global MPT estimation from an arc environment by using porphyritic ultramafic dikes showing flow differentiation, which have records of the chemical composition of the primitive magma, including its water content, because of their high pressure (~0.6 GPa) intrusion and rapid solidification. The solidus conditions for the primary magmas are estimated to be ~1450 °C, ~5.3 GPa. Geochemical data of the dikes show passive upwelling of a depleted mantle source in the garnet stability field without a strong influence of slab-derived fluids. These results, combined with the extensive fluxed melting of the mantle wedge prior to the dike formation, indicate sudden changes of the melting environment, its mechanism, and the mantle source from extensive fluxed melting of the mantle wedge to decompressional melting of the sub-slab mantle, which has been most plausibly triggered by a slab breakoff. The estimated MPT of the sub-slab mantle is ~1350 °C, which is very close to that of the current upper mantle and may reflect the global value of the upper mantle at ~500 Ma if small-scale convection maintained the shallow sub-slab mantle at a steady thermal state. We, therefore, conclude that the Cambrian-Ordovician ophiolite pulse is not attributable to the high temperature of the upper mantle. Frequent occurrence of slab breakoff, which is suggested by our geochemical compilation of Cambrian-Ordovician ophiolites, and subduction termination, which is probably related to the assembly of the Gondwana supercontinent, may be responsible for the ophiolite pulse.

scholarly journals A MODEL OF THE EVOLUTION OF THE LITHOSPHERE OF THE HIMALAYAN-TIBETAN OROGEN

2021 ◽  
pp. 89-107
Author(s):  
R.S. Alekseev, ◽  
◽  
Yu.L. Rebetsky ◽  
Keyword(s):  
Upper Mantle ◽  
Subduction Zones ◽  
Oceanic Lithosphere ◽  
Indian Plate ◽  
Tibet Plateau ◽  
Small Scale ◽  
Continental Lithosphere ◽  
Principal Stresses ◽  
Vertical Movements ◽  
Reference Parameters

The Himalayan-Tibetan orogen is one of the active orogens on Earth. The processes caused by the collision of two continents have attracted attention of many researchers, and over the past decades, a large amount of geological and geophysical data has accumulated, on which models of the evolution of the region are based. The paper presents a model of the evolution of the Tibet plateau and the adjacent mountain chains, which complies with the modern concepts of the structure of the crust. The reference parameters of this model are the data on the values of stresses and on the patterns of the spatial distribution of principal stresses obtained in our own tectonophysical studies in region, as well as in other intracontinental orogens and in subduction zones between lithospheric plates. The basic assumptions of the model are the factors of the long stage of the Indian plate underthrusting beneath the Eurasian continent, metamorphic processes in the submerged slab (oceanic lithosphere) and in the continental lithosphere above it, combination of absolute horizontal movements of the Eurasian and Indian plates, small-scale convection in the upper mantle and vertical movements of matter, both in the continental lithosphere itself and in the upper mantle.

Probing 1-D electrical anisotropy in the oceanic upper mantle from seafloor magnetotelluric array data

2020 ◽  
Vol 222 (3) ◽  
pp. 1502-1525
Author(s):  
Tetsuo Matsuno ◽  
Kiyoshi Baba ◽  
Hisashi Utada
Keyword(s):  
Upper Mantle ◽  
Azimuthal Anisotropy ◽  
Electrical Anisotropy ◽  
Small Scale ◽  
Northwestern Pacific ◽  
Horizontal Distance ◽  
Array Data ◽  
D Model ◽  
Rotational Invariants ◽  
Correction Equations

SUMMARY Electrical anisotropy in the oceanic upper mantle can only be imaged by seafloor magnetotelluric (MT) data, and arguably provides important clues regarding the mantle structure and dynamics by observational determinations. Here, we attempt to probe the electrical (azimuthal) anisotropy in the oceanic mantle by analysing recent seafloor MT array data from the northwestern Pacific acquired atop 125–145 Ma seafloor. We propose a method in which an isotropic 1-D model is first obtained from seafloor MT data through an iterative correction for topographic distortions; then, the anisotropic properties are inferred as deviations from the isotropic 1-D model. We investigate the performance of this method through synthetic forward modelling and inversion using plausible anisotropic 1-D models and the actual 3-D bathymetry and topography of the target region. Synthetic tests reveal that the proposed method will detect electrical anisotropy in the conductive upper mantle or electrical asthenosphere. We also compare the performance of the proposed scheme by using two rotational invariant impedances and two topographic correction equations. The comparison reveals that using different rotational invariants and correction equations provides relatively consistent results, but among the rotational invariants, the sum of squared elements (ssq) impedance yields better recovered results for topographically distorted data than the determinant impedance. An application of the method to seafloor MT array data sets from two areas in the northwestern Pacific reveals the possible presence of two layers of electrical anisotropy in the conductive mantle (<100 Ω-m) at depths of ∼60–200 km. The anisotropy is estimated to be more intense in the shallower layer for both areas. On the other hand, the estimated anisotropic azimuth (defined as the most conductive direction) and the depth to the interface between the two layers are different between the two array areas separated by a small horizontal distance of ∼1000 km in spite of their similar seafloor ages. The most conductive directions are aligned neither with the current absolute plate motion direction nor with the fastest direction of seismic azimuthal anisotropy. The inferred electrical anisotropy features may result from array-scale (∼1000 km) mantle dynamics, such as small-scale convection, which might affect the electrical and seismic properties differently, although there remains the possibility that some portions of these features are explained by laterally heterogeneous mantle structures.

Numerical simulation of asthenospheric flow around cratonic keels

2020 ◽  
Author(s):  
Edgar Santos ◽  
Victor Sacek
Keyword(s):  
Upper Mantle ◽  
Numerical Models ◽  
Relative Displacement ◽  
Planetary Science ◽  
Small Scale ◽  
Time Interval ◽  
Mantle Flow ◽  
Continental Margins ◽  
Thermochemical Convection ◽  
Synthetic Models

<p>In this work, we studied the mantle flow around cratonic keels using numerical models to simulate the thermochemical convection in the terrestrial mantle taking into account the relative displacement between the lithosphere and asthenosphere. The numerical simulations were performed using the finite element code developed by Sacek (2017) to solve the Stokes Flow for an incompressible Newtonian fluid. Several synthetic models in 2D and 3D were constructed considering different keel geometries and different regimes of relative displacement between the lithosphere and asthenosphere. In the present numerical experiments, we adopted a rheology in which the viscosity of the mantle is controlled by temperature, pressure and composition, assuming that the cratonic keel is compositionally more viscous than the surrounding asthenosphere, using a factor f to rescale the lithospheric viscosity compared to the asthenospheric one. We tested different f values, reference viscosity for the asthenosphere, and relative velocity between the lithosphere and the base of the upper mantle, quantifying the amount of deformation of the cratonic keel in each scenario. In general, we conclude that for a relatively low compositional factor (f < 20), the lithospheric keel can be significantly deformed in a time interval of few tens of million years when the lithosphere is moving horizontally relative to the base of the upper mantle, does not preserving its initial geometry. The synthetic models can be helpful for a better understanding of the interaction in the lithosphere-asthenosphere interface such as the deformation and flow patterns in the mantle around the keels, the rate of erosion of the root of the continental lithosphere due to the convection in the upper mantle and how it affects the thermal flow to the surface.</p><p>Sacek, V. (2017). Post-rift influence of small-scale convection on the landscape evolution at divergent continental margins. Earth and Planetary Science Letters, 459, 48-57.</p>

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