Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.
TL;DR: It is suggested that depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock), which allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface.
Abstract: Stable continental regions, free from tectonic activity, are generally found only within ancient cratons—the centres of continents which formed in the Archaean era, 4.0–2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium–osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling.
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TL;DR: The zonation of trace elements in Cr-pyrope has been analyzed to investigate histories recorded by garnet in cool mantle and implications for processes in the mantle wedge during low-angle subduction.
4 citations
Dissertation•
01 Jan 2002
TL;DR: In this article, the authors explore the geophysical structure and evolution of the Australian continental lithosphere and demonstrate that the depth of continental high wave speed anomalies does not universally increase with age, but is dependent on the scale and the tectonic history of the region under consideration.
Abstract: In this thesis, I explore the geophysical structure and evolution of the Australian continental lithosphere. I combine insights from isotropic and anisotropic seismic surface-wave tomography with an analysis of the anisotropy in the mechanical properties of the lithosphere, inferred from the coherence between gravity anomalies and topography. With a new high-resolution waveform tomographic model of Australia, I demonstrate that the depth of continental high wave speed anomalies does not universally increase with age, but is dependent on the scale and the tectonic history of the region under consideration. I construct an azimuthally anisotropic three-dimensional model of the Australian upper mantle from Rayleigh-wave waveforms. I compare Bayesian inverse methods with discretely parameterized regularization methods, and explore the use of regular, tectonic and resolutiondependent tomographic grids. I advocate the use of multitaper spectral estimation techniques for coherence analysis of gravity and topography, applied to Australian isostasy. I investigate the importance of internal loading, the directional anisotropy of the gravitational response to loading, and the estimation bias affecting the long wavelengths of the coherence function. I develop a method for non-stationary coherence analysis which enables a complete characterization of continental strength by the dependency of gravity-topography coherence on wavelength, direction and geologic age. Combining high-resolution, depthdependent anisotropy measurements from surface-wave tomography with the mechanical anisotropy from gravity/topography coherence, I assess the validity of two competing theories regarding the cause of continental anisotropy (vertically coherent deformation or simple asthenospheric flow) quantitatively for the very first time.
4 citations
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..., Lees and Park, 1995]. The original maximum entropy approach of estimating spectra is a parametric method which assumes autoregressive signals [Percival and Walden, 1993]. Following Burg [1975], maximum entropy spectral analysis is based on choosing the spectrum which corresponds to the most random or the most unpredictable time series whose autocorrelation function agrees with the known values....
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Dissertation•
01 Jan 2010
TL;DR: Liu et al. as discussed by the authors presented a 3D shear velocity (Vs) model of the lithospheric structure beneath the Colorado Plateau and its surrounding regions to understand the complicated lithosphere modifications caused by major Cenozoic tectonic and magmatic activities.
Abstract: Imaging Lithospheric Structure Beneath the Colorado Plateau and its Adjacent Regions using Rayleigh Wave Tomography by Kaijian Liu This thesis presents a new 3-D shear velocity (Vs) model of the lithospheric structure beneath the Colorado Plateau and its surrounding regions to understand the complicated lithospheric modifications caused by major Cenozoic tectonic and magmatic activities. Prior to this work, lack of an overall velocity model had prevented a comprehensive understanding of the current encroachment patterns near the margins, and of the perplexing crust/mantle boundary beneath the plateau. Using the new USArray data, I have inverted the isotropic Vs model from the Rayleigh wave dispersion curves obtained by a modified two-plane wave method. The resulting Vs structures not only clearly image the sublithospheric low-velocity channels, but also identify the high velocity anomalies underlying the western plateau for the first time in surface wave tomography to support the recent delamination hypothesis. This work also provides a high-resolution velocity model for geodynamic modeling on lithosphere-asthenosphere interactions.
3 citations
Dissertation•
15 Apr 2014TL;DR: In this paper, the merging of lead, Neodymium, and Hafnium isotopic systems with seismology is a powerful tool to study the formation and deformation of the continental lithosphere.
Abstract: Continental lithosphere is physically and chemically segmented The mapping at a continent size scale of radiogenic isotopes from plutonic acid rocks, sampling the continental crust, and from mafic lavas, sampling the mantle, has similarities with the seismic mapping of the underlying lithosphere These similarities allow to interpret the seismic observations by studying their chemical characteristics and age Lead isotopes are used to date and identify the stacking of crustal segments that form the crust The deviation of Lead model ages with other system is used to identify crustal recycling and the heating of the crust above the Lead-Lead system closing temperature The Lead-Lead system also give access to the under-studied Th/U ratio that constrains the depth of the continental rock sources Somes samples from the Western US are coming from the lower crust, formed by crustal extension or crustal flowing within the crust Neodymium and Hafnium isotopes identify the fusion of an enriched lithospheric mantle under the Colorado Plateau, a place where lithospheric delamination has been observed This mantle melts by adiabatic decompression due to localized expansion or asthenospheric upwelling caused by secondary convection At the end, the merging of Lead, Neodymium, and Hafnium isotopic systems with seismology is a powerful tool to study the formation and deformation of the continental lithosphere
3 citations
Cites background from "Preservation of ancient and fertile..."
...…la faible profondeur d’équilibration des péridotites et le fort appauvrissement par extraction de magmas montre qu’il s’agit du780 manteau lithosphérique sub-continental qui a été stabilisé dans le temps par sa diminution de densité suite aux extractions successives des magmas (Lee et al., 2001)....
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...Une datation des enclaves ultrabasiques est possible par la méthode isotopique Re-Os (Lee et al., 2001; Becker et al., 2006), et les âges sont comparables à l’âge Protérozoïque du socle du CP. L’âge ancien des enclaves, la faible profondeur d’équilibration des péridotites et le fort appauvrissement…...
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...Diffusion of osmium in pyrrhotite and pyrite: implications for closure of the Re-Os isotopic system....
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...Une datation des enclaves ultrabasiques est possible par la méthode isotopique Re-Os (Lee et al., 2001; Becker et al., 2006), et les âges sont comparables à l’âge Protérozoïque du socle du CP. L’âge ancien des enclaves, la faible profondeur d’équilibration des péridotites et le fort appauvrissement par extraction de magmas montre qu’il s’agit du780 manteau lithosphérique sub-continental qui a été stabilisé dans le temps par sa diminution de densité suite aux extractions successives des magmas (Lee et al., 2001)....
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...manteau lithosphérique sub-continental qui a été stabilisé dans le temps par sa diminution de densité suite aux extractions successives des magmas (Lee et al., 2001)....
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References
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TL;DR: In this paper, the authors compared the relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites and found that the most consistent composition of the Earth's core is derived from the seismic profile and its interpretation, compared with primitive meteorites, and chemical and petrological models of peridotite-basalt melting relationships.
10,830 citations
"Preservation of ancient and fertile..." refers background in this paper
...For internal consistency, the density for convecting upper mantl...
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2,058 citations
TL;DR: In this article, the Wilson cycle is used to balance the tectosphere by depleting the continental upper mantle in a basalt-like component, which stabilizes the old continental nuclei against convective disruption.
Abstract: Beneath the old continental nuclei are thick root zones which translate coherently during plate motions. These zones are apparently stabilised against convective disruption by the depletion of the continental upper mantle in a basalt-like component. Construction of this delicately balanced tectosphere is accomplished by the dynamic and magmatic processes of the Wilson cycle.
770 citations
"Preservation of ancient and fertile..." refers background in this paper
...This allows the depleted mantle to form a thicker thermal boundary laye...
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TL;DR: Isotope analyses of Os, Sr, Nd, and Pb elements were caried out on twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae, and Premier kimberlites of southern Africa, to investigate the timing and the nature of melt extraction from the continental lithosphere and its relation to the continent formation and stabilization.
688 citations
"Preservation of ancient and fertile..." refers background in this paper
...Assuming that partial melting leads to stabilization of the lithospheric mantle, the Re–Os isotope systematics of peridotite xenoliths (samples of the lithospheric mantle) can be used to date this time of stabilization; this is because partial melting fractionates Re/Os (Re is moderately depleted and Os is sequestered in the residu...
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TL;DR: In this article, three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e nd values with increasing distance southeastward from the Archean craton.
Abstract: Initial Nd isotopic ratios of crystalline rocks from an area of ∼ 1.5 × 10 6 km 2 of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. The use of three representative rock suites of different ages— Mesozoic and Tertiary peraluminous granitic rocks, middle Proterozoic (ca. 1.4 Ga) “an-orogenic” granitic rocks, and lower Proterozoic (ca. 1.7 Ga) igneous and metamorphic rocks—allows the ages of the provinces to be distinguished on the basis of different Nd isotopic evolution paths rather than solely on the basis of model ages. Three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e Nd values with increasing distance southeastward from the Archean craton. Province 1 is composed of crustal rocks of central Utah and northeastern Nevada, which are characterized by average values of e Nd (1.7 Ga) ≈ 0 and T DM ≈ 2.0–2.3 Ga. Province 2 covers Colorado, southern Utah, and northwestern Arizona and has e Nd (1.7 Ga) ≈ +3 and T DM ≈ 1.8–2.0 Ga. Province 3, which comprises the basement rocks of New Mexico and southern Arizona, has e Nd (1.7 Ga) ≈ +5 and T DM ≈ 1.7–1.8 Ga. An additional region of province 1-type isotopic characteristics, herein named “Mojavia,” is found in eastern California and western Nevada. Crust formation in each province involved a large component of mantle-derived material plus a moderate amount (∼20%) of pre-existing crust. As the new crust was built outward from the Archean nucleus, however, contributions of Archean material to the newly forming crust were more effectively screened, so that the most distal province (3) is derived almost entirely from Proterozoic mantle. The province boundaries are subparallel to the crystallization age trends determined by other workers. An exception to this is the Mojavia region of province 1, which crosscuts and truncates the other provinces in the region of the lower Colorado River. This region appears to be displaced relative to other areas of the North American basement that have similar isotopic characteristics. This suggests the presence of a previously unrecognized large-scale, left-lateral, north-south–trending basement offset of Proterozoic age in the vicinity of the California-Arizona border.
518 citations
"Preservation of ancient and fertile..." refers background in this paper
...But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stabl...
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