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: A model of the three-dimensional shear wave velocity structure under the southwestern United States constructed from the inversion of fundamental mode Rayleigh wave dispersion is presented in this paper.
Abstract: Surface wave tomography of the western United States reveals the presence of an unusually slow shear wave velocity anomaly beneath a region in the Basin and Range Province, sometimes termed the amagmatic zone, which is renowned for its lack of volcanic activity. We present a model of the three-dimensional shear wave velocity structure beneath the southwestern United States constructed from the inversion of fundamental mode Rayleigh wave dispersion. The abundance of data from the deployment of the USArray (a component of the EarthScope project) provides unprecedented resolution. There is an excellent correlation between the location of slow shear wave velocity anomalies and recent (younger than 1 Ma) volcanism. This correlation and the unusually low values of absolute shear velocity beneath the magmatic gap lead us to conclude that there is melt present beneath this region. Previous studies have proposed that the preservation of cold continental lithosphere beneath the magmatic gap accounted for the lack of volcanism; however, this hypothesis is inconsistent with anomalously slow shear wave velocities in the 50–100 km depth range. Perhaps the prevalence of low-angle normal faulting in this region has made it difficult for the melt to escape the mantle. Beneath an immediately adjacent region, the southwestern Nevada volcanic field, where volcanism was abundant 10–15 m.y. ago but has since waned, significantly higher shear velocities suggest that the melt has been extracted from the mantle.
44 citations
TL;DR: In this article, the authors provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15,500 P-to-S receiver functions recorded at 72 USArray and other broadband seismic stations on the southwestern Colorado Plateau (CP) and the southern BRP.
Abstract: Over the past several decades, contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long-term stability of the Colorado Plateau (CP) and crustal thinning beneath the Basin and Range Province (BRP) in the southwestern United States. Here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15,500 P-to-S receiver functions recorded at 72 USArray and other broadband seismic stations on the southwestern CP and the southern BRP. Our results reveal that the BRP is characterized by a thin crust (28.2 ± 0.5 km), a mean Vp/Vs of 1.761 ± 0.014 and a mean amplitude (R) of P-to-S converted wave (relative to that of the direct P wave) of 0.181 ± 0.014 that are similar to a typical continental crust, consistent with the model that the thin crust was the consequence of lithospheric stretching during the Cenozoic. The CP is characterized by the thickest crust (42.3 ± 0.8 km), largest Vp/Vs (1.825 ± 0.009) and smallest R (0.105 ± 0.007) values in the study area. In addition, many stations on the CP exhibit a clear arrival before the P-to-S converted phase from the Moho, corresponding to a lower crustal layer of about 12 km thick with a mafic composition. We hypothesize that the lower crustal layer, which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines, contributed to the long-term stability and preuplift low elevation of the Colorado Plateau.
43 citations
Cites background from "Preservation of ancient and fertile..."
...[55] The most trivial explanation for the stability is that the CP has a mechanically strong (i.e., cold and thick) lithosphere [Blackwell et al., 1991; Lee et al., 2001]....
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...[55] The most trivial explanation for the stability is that the CP has a mechanically strong (i.e., cold and thick) lithosphere [Blackwell et al., 1991; Lee et al., 2001 ]....
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TL;DR: This paper measured whole rock and mineral major element, trace element, and isotopic compositions of spinel-peridotite xenoliths erupted along the central axis of the rift (Elephant Butte) and the eastern margin of the Colorado Plateau (Cerro Chato) to determine their depth of origin and mantle provenance and to test the delamination hypothesis.
43 citations
TL;DR: In this article, detrital zircons from the 9-12 kbar Swakane Gneiss yielded a U-Pb crystallization age of 68.36 ± 0.07 Ma, indicating that the gneiss was deeply buried within 5 Myr of deposition.
Abstract: [1] The metamorphic core of the North Cascades largely comprises island arc and oceanic terranes juxtaposed prior to circa 96 Ma magmatism. However, the tectonic affinity of the structurally deepest terrane, the 9–12 kbar Swakane Gneiss, is distinctly different from other terranes in the core; it is not intruded by arc-related plutons and contains abundant Precambrian zircons. New U-Pb analyses of detrital zircons from the Swakane Gneiss yield dates from 73 Ma to 1610 Ma with a dominant Late Cretaceous population. These data indicate that the Swakane protolith was deposited as late as 72.5 ± 0.6 Ma, the 206Pb/238U date of the youngest detrital grain. Following deposition, the gneiss was intruded by peraluminous leucogranite sheets that may represent partial melt derived from the gneiss at near- to postpeak P-T conditions. One sheet yielded a U-Pb crystallization age of 68.36 ± 0.07 Ma, which indicates that the gneiss was deeply buried within 5 Myr of deposition. Two possible mechanisms considered for this rapid burial (∼7 mm/yr) are overthrusting of a forearc or back arc basin by older crystalline rock or underthrusting of trench sediments during low-angle subduction. The model involving overthrusting of a forearc or back arc basin is most consistent with thermobarometric and isotopic data and the regional geologic setting of the Cascades core. Rapid burial of the Swakane protolith is coincident with burial of sediments that formed the Pelona, Orocopia and Rand schists of southern California; however, differences in peak metamorphic temperatures indicate that conditions of burial must have varied along the plate margin.
41 citations
TL;DR: In this article, the authors used 2D numerical models with 150°C higher mantle temperatures compared to present day conditions to investigate physical and petrological controls of Precambrian orogeny during forced continental plates convergence.
41 citations
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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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