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: In this paper, the authors summarize the post-late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system, and concur with previous estimates of 150 to 180 km of shortening during a brief time span ( 3 cm/yr).
Abstract: We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare-up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo-depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180 km of shortening during a brief time span ( 3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion-related trench sediment was shuffled from the trench to the sub-arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high-pressure/low-temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite-facies metamorphism and limited partial melting.
69 citations
Cites background from "Preservation of ancient and fertile..."
...2001 ) which based on re‐depletion ages were interpreted by the authors to represent lithospheric mantle fragments of Late Archean – Early Proterozoic ages....
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...…presented here (and in numerous other papers) is the presence of a few peridotite xenoliths from the Cima Volcanic Field in southeastern California (Lee et al. 2001) which based on re-depletion ages were interpreted by the authors to represent lithospheric mantle fragments of Late Archean – Early…...
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TL;DR: In this article, the nature of PGE-Re (PGE = Pt, Pd, Os, Ir, Ru, and Ru/Ir) behavior in subcontinental lithospheric mantle was investigated using new, high precision PGERe abundance measurements and previously published Re-Os isotopic analyses of peridotite xenoliths.
67 citations
Cites background or methods from "Preservation of ancient and fertile..."
...Re-Os isotopic systematics have already been reported and discussed in Lee et al. (2000, 2001b) but are shown here for completeness (Table 1; Fig....
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...Although similar to the other Big Creek peridotites in terms of major elements and silicate petrography (Lee et al., 2001a), sample 1026V from Big Creek is anomalous because it has abundantly visible sulfides (including the presence of chalcopyrite; Figs....
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...Equilibration temperatures range between 1000 to 1100°C (Lee, 2001; Lee et al., 2001b)....
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...This is evidenced by olivine Mg#s (Mg# molar Mg/(Mg Fe)), ranging from 0.886 (fertile) to 0.914 (depleted), and negative correlations between olivine forsterite content and the amount of TiO2 and Na2O in clinopyroxene (Lee et al., 2001a)....
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...Mass spectrometry details for Os isotopic analyses determined by thermal ionization mass spectrometry have been described in detail elsewhere (Lee et al., 2000, 2001b; Lee, 2001)....
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TL;DR: In this article, the average initial 187Os/188Os compositions of unaltered chromites of massive chromitite layers from two mining districts are 0.1031−+/−−0.0007 (Nuasahi) and 0.0005 (Sukinda), with negative γOs values of − 1.78 −1.65 and − 2.04 −2.43, respectively.
65 citations
TL;DR: In this article, the authors present constraints on the regional variations of the seismic and mechanical thickness of the Australian lithosphere and use the wavelength at which the coherence of surface topography and Bouguer gravity drops below half of its long-wavelength maximum.
Abstract: [1] We present constraints on the regional variations of the seismic and mechanical thickness of the Australian lithosphere We infer the seismic thickness from a waveform tomographic model of S-wave speed, and as a proxy for the elastic thickness we use the wavelength at which the coherence of surface topography and Bouguer gravity drops below half of its long-wavelength maximum Our results show that on scales <1000 km the relationship between the age of the crust and the thickness of the lithosphere is more complicated than longer-wavelength or global averages suggest Recent geochemical and geodynamical evidence for small-scale secular variations of the composition and stability of continental cratons further illustrates the complexity of the age dependence of seismo-mechanical lithospheric properties on regional scales
64 citations
TL;DR: In this paper, the authors estimated the minimum volume of mantle needed to generate the mafic magmas parental to the preserved mid-Tertiary igniferous rocks in the Sierra Madre Occidental of northern Mexico.
63 citations
Cites background from "Preservation of ancient and fertile..."
...... Plateau xenoliths, in fact, have now provided direct evidence that refrigeration and hydration of the lithospheric mantle did take place in the vicinity of the present-day southern Rocky Mountains, that this hydration was Late Cretaceous to Early Tertiary in age but affected Proterozoic age mantle lithosphere, and that the peridotitic portions of the lithosphere were fertile with respect to basalt generation in the mid-Tertiary ( Lee et al., ......
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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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