Thin Mantle Lithosphere Research Articles

Shear‐Velocity Structure and Dynamics Beneath the Sicily Channel and Surrounding Regions of the Central Mediterranean Inferred From Seismic Surface Waves

AbstractThe evolution of the Sicily Channel Rift Zone (SCRZ) is thought to accommodate the regional tectonic stresses of the Calabrian subduction system. Much of the observations we have today are either limited to the surface or to the upper crust or deeper from regional seismic tomography, missing important details about the lithospheric structure and dynamics. It is unclear whether the rifting is passive from far‐field extensional stresses or active from mantle upwelling beneath. We measure Rayleigh‐and Love‐wave phase velocities from ambient seismic noise and invert for 3‐D shear‐velocity and radial anisotropic models. Variations in crustal S‐velocities coincide with topographic and tectonic features. The Tyrrhenian Sea has a ∼10 km thin crust, followed by the SCRZ (∼20 km). The thickest crust is beneath the Apennine‐Maghrebian Mountains (∼55 km). Areas experiencing extension and intraplate volcanism have positive crustal radial anisotropy (VSH > VSV); areas experiencing compression and subduction‐related volcanism have negative anisotropy. The crustal anisotropy across the Channel shows the extent of the extension. Beneath the Tyrrhenian Sea, we find very low sub‐Moho S‐velocities. In contrast, the SCRZ has a thin mantle lithosphere underlain by a low‐velocity zone. The lithosphere‐asthenosphere boundary rises from 60 km depth beneath Tunisia to ∼33 km beneath the SCRZ. Negative radial anisotropy in the upper mantle beneath the SCRZ is consistent with vertical mantle flow. We hypothesize a more active mantle upwelling beneath the rift than previously thought from an interplay between poloidal and toroidal fluxes related to the Calabrian slab, which in turn produces uplift at the surface and induces volcanism.

Crust and upper mantle structure beneath Southwest China and its implications for Mesozoic multistage gold deposits

The Youjiang Basin (YB) on the southwestern margin of the Yangtze Craton (YC) contains >200 Triassic or Cretaceous gold deposits, but deep dynamic processes contributing to large-scale gold accumulation remain unknown. Based on a south–north trending linear broadband seismic array, we obtained the crust and mantle structure beneath the YB and southwestern YC by P-wave and S-wave receiver functions and ambient noise tomography. Our results show a remarkable lithospheric boundary beneath the northern margin of the YB, which separates the YB with felsic thin crust and thin mantle lithosphere from the YC with both a thick crust and thick mantle lithosphere. A new structural image shows that the southern end of the YC Moho dips southward and overlaps with the YB Moho. A 5-km-thick high-velocity layer is also found at a depth of 10 km beneath the northern YB. Combining this information with previous tectonic and geological results, we reveal the process of Mesozoic lithospheric evolution in the southwestern South China Block. We infer that both the pre-Mesozoic weak crust and mantle lithosphere and the intense intracontinental orogenesis in the Early-Middle Triassic contributed to the formation of first-stage Au deposits in the YB. Lithospheric delamination in the Early Cretaceous thinned the lithosphere of YB rather than YC, forming a steep lithosphere–asthenosphere boundary beneath the transition zone between YB and YC, which favored upwelling of Au-rich asthenosphere below the thick cratonic root to the bottom of the crust in the northern YB and contributed to the second-stage Au deposits in the YB.

The lithospheric structure beneath the northeastern Tibetan Plateau and western North China Craton from S receiver function imaging and its implications for lateral growth of the plateau

The mantle lithospheric structures beneath the northeastern Tibetan Plateau (NETP) and adjacent areas provide critical information about the role of the mantle lithosphere in the lateral growth of the plateau. In this study, we construct a comprehensive image of the lithospheric structure across the NETP and western North China Craton (NCC) using the S-wave receiver function method based on the dense seismic array of ChinArray II. We detect one or multiple negative conversions at the depth ranging from 80 to 200 km across most of the study region, which is associated with the seismic lithosphere-asthenosphere boundary (LAB) or mid-lithospheric discontinuity (MLD) within the lithospheric mantle. The main features include that the Beishan Block, West Qinling Orogen, and Qilian Orogen present a relatively shallow LAB at a depth range of 120–140 km, while the Longxi Basin and Alashan Block have the similar lithospheric structure with a deep LAB at depth > 150 km and some MLDs within the lithospheric mantle. We find that the Qilian Orogen in the western NETP overlying a thin mantle lithosphere has been significantly elevated, whereas the Longxi Basin in the eastern NETP with a thick mantle lithosphere lacks both high elevations and significant crustal thickening. It implies that the thick mantle lithosphere beneath the eastern NETP and western NCC likely prevents the plateau from invading the Longxi Basin, while the thin mantle lithosphere across the western NETP and Beishan Block favors outward growth of the Qilian Orogen. Our seismic images provide new insights into the lithospheric heterogeneities and their roles in the lateral growth of the Tibetan Plateau.

Grain‐Boundary Diffusion Creep of Olivine: 2. Solidus Effects and Consequences for the Viscosity of the Oceanic Upper Mantle

AbstractIn Part 1 of this study (Yabe et al., 2020, https://doi.org/10.1029/2020JB019415), we established an olivine grain‐boundary diffusion creep law that incorporates the chemically induced enhancement of creep rates, which becomes significant with increasing temperature. The effect was attributed to grain‐boundary disordering that starts at ~0.92 × solidus (bulk eutectic temperature). In this study, we estimated solidus temperatures of the samples used in the previous diffusion creep experiments. These temperatures were used to compare previously reported diffusion creep rates for olivine with our established diffusion creep law. We found that the law explains a difference of up to 2 orders of magnitude in olivine creep rates at the same temperatures, stresses, grain sizes, and water contents in the previous studies. The geotherm normalized by the mantle solidus was calculated for the upper mantle with water contents ranging from 0 to 300 μg/g, which predicts depths where grain‐boundary diffusion creep is enhanced due to grain‐boundary disordering. Constructed viscosity‐depth profiles reveal a very thin mantle lithosphere beneath mid‐ocean ridges, with development of the lithosphere away from the ridge, leaving a low‐viscosity region below. Given a grain size of 1 mm and depending on the water content, a viscosity of 2–5 × 1019 Pa·s is predicted for the low‐viscosity mantle beneath 50‐million‐year‐old seafloor.

ArcCRUST: Arctic Crustal Thickness From 3‐D Gravity Inversion

AbstractThe ArcCRUST model consists of crustal thickness and estimated crustal thinning factors grids for the High Arctic and Circum‐Arctic regions (north of 67°N). This model is derived by using 3‐D forward and inverse gravity modeling. Updated sedimentary thickness grid, an oceanic lithosphere age model together with inferred microcontinent rifting ages, variable crystalline crust and sediment densities, and dynamic topography models constrain this inversion. We use published high‐quality 2‐D seismic crustal‐scale models to create a database of Depths to Seismic Moho (DSM) profiles. To check the quality of the ArcCRUST model, we have performed a statistical analysis of misfits between the ArcCRUST Moho depths and DSM values. Systematic analysis of the misfits within the Arctic sedimentary basins provides information about tectonic processes unaccounted by the assumed model of pure‐shear lithospheric extension. In particular, our model implies a less dense and/or thin mantle lithosphere underneath microcontinents in the deep Arctic Ocean where the ArcCRUST depth to Moho values exceed the DSM. A systematically larger gravity‐derived crustal thickness (~3 km) under the western and northern Greenland Sea points to a hotter upper mantle implied by the seismic tomography models in the North Atlantic.

A new crustal model of the Anatolia–Aegean domain: evidence for the dominant role of isostasy in the support of the Anatolian plateau

The engines of surface deformation in the Anatolia-Aegean region are a matter of debate, including the origin of the high elevations of the Anatolian plateau. Recent publications based on geological and thermomechanical modelling emphasize the role of dynamic topography in the plateau uplift. However, quantitative estimates of the contribution of dynamic topography are affected by large uncertainties due to insufficient knowledge of the crustal structure, in particular crustal thickness and density. To reduce these uncertainties, we provide a new accurate crustal thickness map of the Anatolia-Aegean domain computed from a large volume of broadband seismic data. In addition, we display high-resolution seismic sections of the internal structure of the crust in Western and Central Anatolia. Density contrasts are derived from the same seismic data set and Bouguer gravity anomaly computed from the EGM2008 model. Our crustal thickness model is highly correlated with the topography suggesting that the Anatolian plateau is close to isostatic equilibrium. The average density difference between crust and upper mantle computed from our crustal model and Bouguer gravity anomaly is low compared to the global average, ∼0.315 ×10 3 kg m −3. The ratio of surface elevation to crustal thickness is lower than average, 1/9.4, which also indicates a low-density crust. Differences between isostatic topography and observed topography are overall small (<500 m). The east-to-west gradients of crustal thickness and topography changes are nearly constant in between the Taurides and Pontides at the northern and southern borders of Anatolia. The observed constant crustal thickness gradient may indicate a low viscosity lower crust supported by the thin mantle lithosphere evidenced by seismic tomography beneath the Anatolian plateau. We propose that viscous flow in the lower crust has smoothed out lateral changes in the crustal structure expected for such a heterogeneous collage of continental fragments. This flow may originate from gravitational potential energy differences between Eastern Anatolia (thick crust, high elevations) and the Aegean Sea (thin crust, low elevations), suggesting that gravity plays an integral part in the westward escape of Anatolia.

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

AbstractWe report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (&lt;29 ± 8 Ma) suggests that a thin mantle lithosphere (&lt;80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane.

Receiver function analysis of the crust and upper mantle in Fennoscandia – isostatic implications

The mountains across southern Norway and other margins of the North Atlantic Ocean appear conspicuously high in the absence of recent convergent tectonics. We investigate this phenomenon with receiver functions calculated for seismometers deployed across southern Fennoscandia. These are used to constrain the structure and seismic properties of the lithosphere and primarily to measure the thickness and infer the bulk composition of the crust. Such parameters are key to understanding crustal isostasy and assessing its role, or lack thereof, in supporting the observed elevations. Our study focuses on the southern Scandes mountain range that has an average elevation >1.0 km above mean sea level. The crust–mantle boundary (Moho) is ubiquitously imaged, and we occasionally observe structures that may represent the base of the continental lithosphere or other thermal, chemical, or viscous boundaries in the upper mantle. The Moho resides at ∼25–30 km depth below mean sea level in southeastern coastal Norway and parts of Denmark, ∼35–45 km across the southern Scandes, and ∼50–60 km near the Norwegian–Swedish border. That section of thickest crust coincides with much of the Transscandinavian Igneous Belt and often exhibits a diffuse conversion at the Moho, which probably results from the presence of a high wave speed, mafic lower crust across inner Fennoscandia. A zone of thinned crust (<35 km) underlies the Oslo Graben. Crustal Vp/Vs ratio measurements show trends that generally correlate with Moho depth; relatively high Vp/Vs occurs near the coast and areas affected by post-Caledonide rifting and lower Vp/Vs appears in older, unrifted crust across the southern Scandes. Our results indicate that most of the observed surface elevation in the southern Scandes is supported by an Airy-like crustal root and potentially thin mantle lithosphere. To the east, where thicker crust and mantle lithosphere underlie low elevations, the presence of dense mafic lower crust fits a Pratt-like model for isostatic compensation. Because the Scandes mountains occupy the location of the ancient Caledonian orogeny, which created presumably much thicker crust and lithosphere by ca. 400 Ma, much of the dense lower crust or mantle lithosphere that is expected to form beneath large mountain belts must have been removed sometime afterwards to instill the current lithospheric architecture that underlies the region.

Lithospheric cooling and thickening as a basin forming mechanism

Widely accepted basin forming mechanisms are limited to flexure of the lithosphere, and lithospheric stretching followed by cooling and thermal subsidence. Neither of these mechanisms works for a group of large basins, sometimes known as “intracontinental sags”. In this paper we investigate cooling and thickening of initially thin lithosphere as a basin forming mechanism, by a combination of forward modelling and a backstripping study of two Palaeozoic North African basins: Ghadames and Al Kufrah. These are two of a family of basins, once unified, which lie over the largely accretionary crust of North Africa and Arabia. Such accretionary crust tends to be juvenile, consisting of amalgamated island arcs, accretionary prisms and melanges, and typically has near-normal crustal thicknesses but initially thin mantle lithosphere. Post-accretion subsidence is modelled using a plate cooling model similar to cooling models for oceanic lithosphere. The crustal composition and thickness used in the models are varied around average values of accretionary crust to represent likely heterogeneity. The model allows the lithosphere to thicken as it cools and calculates the resulting isostatic subsidence. Water-loaded tectonic subsidence curves from these forward models are compared to tectonic subsidence curves produced from backstripped wells from Al Kufrah and Ghadames Basins. A good match between the subsidence curves for the forward model and backstripping is produced when the best estimates for the crustal structure, composition and the present day thickness of the lithosphere for North Africa are used as inputs for the forward model. The model produces sediment loaded basins of 2–7 km thickness for the various crustal assemblies over ~ 250 Myr. This shows that lithospheric cooling provides a viable method for producing large basins with prolonged subsidence, without the need for initial extension, provided the condition of initially thin mantle lithosphere is met.

Quaternary syn-collision magmatism from the Iran/Turkey borderlands

Quaternary basaltic and andesitic lavas from the NW Iran/eastern Turkey border area are related to the active Arabia–Eurasia collision. The lavas occur within the Turkish–Iranian plateau, which ceased crustal thickening before the establishment of a number of volcanic centres within Iran and eastern Turkey, beginning at ~ 10 Ma. Models for generating syn-collision magmatism in eastern Anatolia have invoked slab-break-off beneath the thick crust and thin mantle lithosphere of the Cenozoic East Anatolia Accretionary Complex and/or partial loss of the lower lithosphere. Here we report geochemical and Sm–Nd/Rb–Sr data from a ~ 200 km long, N–S traverse that samples volcanic flows in NW Iran, many of which originate from centres in Turkey such as Ararat and Tendürek. Samples are transitional alkali/tholeiitic basalts and andesites. Ararat samples have lower Nb, lower large ion lithophile element (LILE) concentrations with 143Nd/ 144Nd ~ 0.51290. Other, volumetrically smaller, centres have higher Nb, higher LILE, with 143Nd/ 144Nd ~ 0.51265. Abundances of LILE and Nb increase from north to south. The presumed degree of partial melting increases in the opposite direction, away from the Arabia–Eurasia suture. Melting is inferred to have taken place in the spinel lherzolite field, largely from a continental lithosphere source influenced by Mesozoic and early Cenozoic Neo-Tethyan subduction, but a separate source with long-term enrichment is needed to explain the high Nb, lower 143Nd/ 144Nd compositions.

When and how did plate tectonics begin? Theoretical and empirical considerations

Plate tectonics is the horizontal motion of Earth’s thermal boundary layer (lithosphere) over the convecting mantle (asthenosphere) and is mostly driven by lithosphere sinking in subduction zones. Plate tectonics is an outstanding example of a self organizing, far from equilibrium complex system (SOFFECS), driven by the negative buoyancy of the thermal boundary layer and controlled by dissipation in the bending lithosphere and viscous mantle. Plate tectonics is an unusual way for a silicate planet to lose heat, as it exists on only one of the large five silicate bodies in the inner solar system. It is not known when this mode of tectonic activity and heat loss began on Earth. All silicate planets probably experienced a short-lived magma ocean stage. After this solidified, stagnant lid behavior is the common mode of planetary heat loss, with interior heat being lost by delamination and “hot spot” volcanism and shallow intrusions. Decompression melting in the hotter early Earth generated a different lithosphere than today, with thicker oceanic crust and thinner mantle lithosphere; such lithosphere would take much longer than at present to become negatively buoyant, suggesting that plate tectonics on the early Earth occurred sporadically if at all. Plate tectonics became sustainable (the modern style) when Earth cooled sufficiently that decompression melting beneath spreading ridges made thin oceanic crust, allowing oceanic lithosphere to become negatively buoyant after a few tens of millions of years. Ultimately the question of when plate tectonics began must be answered by information retrieved from the geologic record. Criteria for the operation of plate tectonics includes ophiolites, blueschist and ultra-high pressure metamorphic belts, eclogites, passive margins, transform faults, paleomagnetic demonstration of different motions of different cratons, and the presence of diagnostic geochemical and isotopic indicators in igneous rocks. This record must be interpreted individually; I interpret the record to indicate a progression of tectonic styles from active Archean tectonics and magmatism to something similar to plate tectonics at ∼1.9 Ga to sustained, modern style plate tectonics with deep subduction—and powerful slab pull—beginning in Neoproterozoic time.

Constraints on the composition and thermal structure of the upper mantle beneath NW Turkey: Evidence from mantle xenoliths and alkali primary melts

Abstract Ultramafic xenoliths entrained in the Late Miocene alkali basalts and basanites from NW Turkey include refractory spinel-harzburgites and dunites accompanied by subordinate amount of spinel-lherzolites. Whole-rock major element and mineral chemical characteristics indicate that the xenoliths are mostly the solid residues of varying degrees of partial melting (∼3 to ∼15%), but some have mineral chemical signatures reflecting processes of melt/rock interaction. The mineral compositions of the undisturbed xenoliths have been used to estimate temperatures and, where possible, pressures of equilibration, and to construct a geothermal framework to interpret the thermal structure of the upper mantle beneath the region at the time of basaltic melt extraction. Most of the peridotite xenoliths have chemically homogeneous mineral phases reflecting ambient equilibrium pressure–temperature conditions. The combination of results from Ca-in-orthopyroxene geothermometer and Ca-in-olivine geobarometer yields pressure–temperature estimates for spinel-peridotites that fall in experimentally determined spinel-lherzolite field in CaO–Fe–MgO–Al 2 O 3 –SiO 2 –Cr 2 O 3 (CFMASCr) system. These pressure–temperature data sets have been used to construct the Late Cenozoic geotherm of NW Turkey, which defines a depth-temperature array from about 1.4 GPa at 810 °C to 2.1 GPa at 1180 °C. Extrapolation of this array to the adiabatic upwelling curve of normal temperature asthenospheric mantle indicates an apparent lithospheric thickness of ∼80 km. The geothermal gradient of NW Turkey is only slightly higher than that of the conventional conductive models (∼90 mW m −2 ) and may be interpreted as being a consequence of the thermal perturbation by the heat input into the relatively thin mantle lithosphere via the passage and emplacement of magma(s). In contrast, the calculations, based on the melt products of experimental phase equilibria, show that the compositions of the host magmas precipitating the most magnesian olivine (with 88.5 to 91.4% forsterite; Mg 2 SiO 4 ) phenocrysts of alkaline melts reveal significantly higher anhydrous liquidus temperatures that range from about 1290 to 1410 °C. The results are inconsistent with melt generation by either deep-seated thermal anomalies associated with mantle plumes ( T p > 1450 °C) or melting of thermally perturbed (wet) mantle lithosphere, but are consistent with the adiabatic upwelling of normal temperature ( T p ∼ 1300 °C) mantle asthenosphere probably along the lithospheric scale major shear zones.

Sierra Nevada uplift: A ductile link to mantle upwelling under the Basin and Range province

Recent geophysical studies in the southern Sierra Nevada found no significant Airy-type crustal root; the mountain range seems largely supported by the buoyant asthenosphere beneath an abnormally thin mantle lithosphere. We suggest that the late Cenozoic uplift of the Sierra Nevada may have resulted from mantle upwelling under the Basin and Range province, which tends to push ductile material within the surrounding lithosphere, causing it to flow away and downward. Numerical modeling indicates that such ductile flow could lead to pronounced lithospheric thinning under the High Sierra and lithospheric thickening under the western Sierra Nevada, comparable to the observed structure.

Collisional plateaus

In the fifteen years since the importance of collisional plateaus with thickened continental crust began to be recognized as one of the inevitable consequences of the processes of plate tectonics, rapid progress in their understanding has come from studies of the world's only active terminal collision zones in the Himalayan-Tibetan and Turkish-Iranian plateaus. Ancient collisional plateaus are being recognized throughout the geological record (back to 3.8 Ga) from the occurrence of extensive areas (typically > 500,000 km 2) of 8 kbar metamorphism in granulite facies or from the occurrence of extensive areas of higher level minimum-melt composition granite rocks whose isotopic signatures indicate reactivation of existing continental crust rather than addition of new crust from the mantle at the time of collision. Recognition of strike-slip faulting in the ancient collisional plateau areas indicates that “tectonic escape” may have been as important in the past as it is today. Earth may not be the only planet on which collisional plateaus are important. The highlands of Venus (approximately 7% of the surface with elevations over 1.5 km above mean planetary radius) can only exist as a result of crustal thickening, and not as a product of lithospheric thinning. Most of these highlands can be explained by models involving volcanic construction. However, the highest peaks, including Maxwell Montes, the highest mapped area of Venus rising over 10 km above mean planetary radius, require much greater crustal thickening to support them than can reasonably be explained by a volcanic mechanism. Geological features of Maxwell Montes inferred from radar images suggest some analogy between Maxwell Montes and the Tibetan plateau. It is somewhat paradoxical that extensional tectonics are commonly associated with continental collision, and that collision-related rifts may be the only sites where the uppermost layers of a collision-thickened crust are preserved from erosion. Extensional stress fields are generated during continental collision, primarily in areas associated with strike-slip faulting and “tectonic escape”. Additional extensional stresses are gravitationally generated associated with the topography and thickened crust in a collision zone. Tectonically thickened crust is particularly susceptible to rifting as its lithosphere is weak as a result of heating associated with magmatism. This lithosphere is also compositionally weak because of the relatively thick crust, dominated by a weak quartz rheology, and thin mantle lithosphere, dominated by a strong olivine rheology, in comparison with a lithosphere with a more normal crustal thickness. Thus, the common association of rifts and collision zones may be a consequence of both stresses generated during collision and modification of the lithosphere by collision.