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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31 Oct 2018
18 citations
TL;DR: In this article, a combination of edge-driven convection on craton margins and asthenospheric flow triggered by rift propagation during the Atlantic and Tethys rifting is suggested to be the main source of heat.
Abstract: The architecture and tectono-magmatic evolution of the lithosphere of Europe are the result of a succession of subduction, rifting and inputs from plumes that have modified the lithospheric mantle since the Neoproterozoic (750–500 Ma). These events gave birth to contrasting crust-mantle and lithosphere-asthenosphere mechanical coupling between strong, viscous, thick, cold, depleted mantle of the Archean lithosphere of the West African Craton and the East European Craton, and the weak, low viscous, thin, hot and less depleted mantle of the Phanerozoic lithosphere of Central Europe. These differences were long-lived and explain the first-order present-day stresses and topography as well as the styles of orogenic deformation. The lack of thermal relaxation needed to maintain rheological contrasts over several hundreds of millions of years requires high mantle heat flux below Central Europe since at least the last 300 Ma. A combination of edge-driven convection on craton margins and asthenospheric flow triggered by rift propagation during the Atlantic and Tethys rifting is suggested to be the main source of heat. The topography of Central Europe remained in part dynamically supported during most of the Mesozoic thinning in line with the long-term stability of thermal-mechanical structure of the lithosphere. Timing and rates of exhumation recorded across Western Europe during convergence indicate that an additional control by the architecture of Mesozoic rifted margins is required. By 50 Ma the acceleration of orogenic exhumation, from the High Atlas to the Pyrenees, occurred synchronously with the onset of extension and magmatism in the West European Rift. Extension marks the onset of distinct orogenic evolution between Western Europe (Iberia) and the Alps (Adria) in the east, heralding the opening of the Western Mediterranean. A major kinematic re-organisation occurred triggering the involvement of more buoyant and thicker portions of rifted margins resulting in widespread orogenic growth. We conclude that conceptual models of collision require to better account for the thermo-magmatic evolution of the continental lithosphere, especially the original architecture and composition of its mantle, as well as the precise knowledge of the architecture of the rifted margins to explain the timing and rates of orogenic topography.
17 citations
TL;DR: Song and Helmberger as discussed by the authors adapted the preferred S wave model and scaled the P wave model using a suite of scaling factors (SF ≡ dlnVs/dlnVp) to obtain corresponding Vp/Vs anomaly and infer the origin of the slab-like anomaly.
Abstract: [1] East of the Rio Grande Rift, tomographic images of teleseismic data have revealed a SE dipping, slab-like structure underneath the western edge of the Great Plains in the southwestern United States. However, finite difference synthetics require an amplified tomographic model to reproduce the waveform distortions as observed in broadband waveform data recorded along the LA RISTRA Transect. In addition to travel time anomalies, Song and Helmberger (2007) demonstrated how to use S waveforms and their amplitude patterns to further constrain the magnitude of the anomalous structure. Their preferred S velocity model suggests that the slab-like structure is about 4% fast, 120 km thick and dipping 70–75° to the SE to about a depth of 600 km. We adapt the preferred S wave model from Song and Helmberger (2007) and scale the P wave model using a suite of scaling factors (SF ≡ dlnVs/dlnVp). We find that synthetics from the P model generated by SF ≈ 1.25 can distort P waveforms and fit the amplitude pattern best. Such a low SF indicates that the slab-like anomaly is not only cold but also compositionally distinct. We make use of SF and the S wave anomaly simultaneously to obtain corresponding Vp/Vs anomaly and infer the origin of the slab-like anomaly. Our result suggests that the observed sub-lithospheric detachment is 310 ± 20°C colder and more depleted than the adjoining mantle asthenosphere by 3 units of Mg# (Mg / (Mg + Fe) × 100). It is negatively buoyant and is geodynamically consistent with the observed foundering of the continental lithosphere at the eastern edge of the Rio Grande Rift. In short, we demonstrate the importance of seismic waveform anomalies in geochemical and geodynamic inferences.
17 citations
Cites result from "Preservation of ancient and fertile..."
...Our estimate of D(Mg#) � 3 in the deep upper mantle between the western Great Plains and the Rio Grande Rift is consistent with xenoliths and xenocrysts analysis on variations in Mg# between the sub-continental lithosphere and the mantle asthenosphere [ Lee et al., 2001; Griffin et al., 2004; O’Reilly and Griffin, 2006]....
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...If the Mg# of the mantle asthenosphere is � 0.88, the Mg# of the sub-lithospheric detachment we observed is � 0.91, which is similar to the estimate for the deeper part of the old continental lithosphere, such as the Kaapvaal Craton, the Slave Craton, and the Colorado Plateau [ Lee et al., 2001; Griffin et al., 1999; Lee, 2006; O’Reilly and Griffin, 2006]....
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TL;DR: The authors analyzed the U-Pb age and Hf isotopic composition of zircons from both the oldest granodiorite plutons and the oldest metasedimentary rocks across a 180 km-long cross-strike transect in Grand Canyon and showed that the characteristically bimodal population of detrital Zircons in the Vishnu Schist (2.5 Ga and 1.8 Ga modes) yields mixed ǫ-Hf(t) values, primarily between +5 to −5, that are uniform across the tran
Abstract: The Paleoproterozoic Mojave and Yavapai crustal provinces in southwestern Laurentia contain evolved and juvenile crust, respectively, but the nature of the province boundary remains uncertain. 1.78–1.35 Ga crystalline basement rocks of the Mojave Province preserve an evolved isotopic signature reflecting an Archean crustal component in several isotopic systems (Nd, Pb, Hf). However, no Archean rocks have been found, and hence the origin and tectonic significance of this Archean component are also unclear. This paper analyzes the U-Pb age and Hf isotopic composition of zircons from both the oldest granodiorite plutons (1.84–1.71 Ga) and the oldest metasedimentary rocks (1.75 Ga Vishnu Schist) across a 180-km-long cross-strike transect in Grand Canyon. This transect crosses the Crystal shear zone, which has been proposed as the location of a suture separating the provinces. Our results show that the characteristically bimodal population of detrital zircons in the Vishnu Schist (2.5 Ga and 1.8 Ga modes) yields mixed ɛHf(t) values, primarily between +5 to –5, that are uniform across the transect. Another new finding is that the 1.84 Ga Elves Chasm pluton, on which the Vishnu Schist was deposited, yields juvenile ɛHf(t) values of +5 to +12 and was not the dominant source for the ca. 1.85 Ga peak in the 1.75 Ga Vishnu Schist. Instead, the Vishnu Schist was derived from an Archean craton mixed with intermediate to evolved 1.85 Ga crust. Metasediments show no evidence in support of the proposed suture. Paradoxically, plutons east and west of the Crystal shear zone do support models for a crustal suture. Plutons east of the Crystal shear zone dated at 1.74–1.71 Ga yield juvenile ɛHf(t) values of +5 to +12 that are characteristic of the Yavapai Province. Plutons west of the Crystal shear zone show juvenile to evolved Paleoproterozoic grains (ɛHf(t) of –5 to +10) as well as xenocrystic Archean and 1.85 Ga grains (ɛHf(t) of –12 to +10). These data support the proposition that the Crystal shear zone marks a sharp boundary between the Mojave and Yavapai crustal provinces. However, the overlapping Vishnu Schist suggests a more complicated crustal architecture. The depositional setting of the Vishnu Schist remains unclear; however, we interpret the ultimate geometry of the transect to reflect an ∼200-km-wide middle-crustal duplex system in which the 1.75 Ga Vishnu Schist was deposited across both Mojave and Yavapai crust. This system was subsequently imbricated in an accretionary complex. The ultimate architecture is of a distributed boundary with slivers of plutons that carry the isotopic signature of their respective provinces imbricated within metasediments.
17 citations
01 Jan 2011
TL;DR: In this article, the authors present new trace-element data for kimberlite-hosted lithospheric peridotites and metasomites and show that neither hydrous nor anhy-drous mantle xenoliths make suitable sources for continental or oceanic basalts.
Abstract: The continental lithospheric mantle (CLM) is a small-volumed (ca. 2.5% of the total mantle), chemically distinct mantle reservoir that has been suggested to play a role in the source of continental and oceanic magmatism. It is our most easily identifi- able reservoir for preserving chemical heterogeneity in the mantle. Petrological and geophysical constraints indicate that the maximum depth of the CLM is ca. 250 km. There is a clear secular variation of CLM composition, such that CLM formed in the last 2 Gyr is less depleted and therefore less dynamically stable than ancient CLM formed in the Archean. We present new trace-element data for kimberlite-hosted lithospheric peridotites and metasomites. These data, combined with other data for spinel peridotites from non-cratonic regions, show that neither hydrous nor anhy- drous lithospheric mantle xenoliths make suitable sources for continental or oceanic basalts. Addition of a hydrous phase, either amphibole or phlogopite, to depleted peridotite results in positive Nb and Ti anomalies that are the opposite of those pre- dicted for some flood-basalt sources on the basis of their trace-element abundances. Overall, the Sr and Nd isotopic composition of cratonic and non-cratonic CLM is close to bulk Earth, with cratonic CLM showing small numbers of extreme com- positions. Thus, while the CLM is certainly ancient in many locations, its average composition is not significantly 'enriched' over primitive upper mantle, in terms of either radiogenic isotopes or trace elements. These characteristics, plus a change in lithospheric chemistry with depth, indicate that the elemental and isotopic compo- sition of lithospheric mantle likely to be re-incorporated into convecting mantle via delamination/thermal erosion processes is probably not very distinct from that of the convecting mantle. These observations lead us to question the requirement for CLM participation in the source of oceanic magmas and to promote consideration of a mantle that is chemically heterogeneous on all scales. Hf and Nd isotope compo- sitions identify a distinctive source component in deeply derived alkaline volcanics associated with continents. This component cannot be constrained to the CLM but may originate from a deeper reservoir of ancient, subducted oceanic crust stored in the mantle.
16 citations
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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