Continental Keel Research Articles

Structure of the crust and upper mantle in Greenland and northeastern Canada: Insights from anisotropic rayleigh-wave tomography

Summary Seismic velocity models provide important constraints on Greenland’s deep structure, which, in turn, has profound implications for our understanding of the tectonic history of this region. However, the resolution of seismic models has been limited by a sparse network, particularly in northern and central Greenland. We address these limitations by generating new high-resolution Rayleigh-wave phase velocity maps encompassing Greenland and northeastern Canada by processing over three decades of teleseismic earthquake records and incorporating recently added stations in Greenland and Arctic Canada. These phase velocity maps are sensitive to structure from the lower crust down to the sub-lithospheric mantle (25-185 s period). We find significant heterogeneity and a strong correlation between isotropic and anisotropic seismic velocities with inferred geological structure. High seismic velocities associated with cratonic lithosphere are broadly divided into two regions, with a belt of reduced velocity spanning central Greenland, which we interpret as lithospheric erosion resulting from interaction between the Greenland continental keel and the Iceland plume. Within each region, we identify tectonic subdivisions that suggest fundamental differences between the blocks that make up Precambrian Greenland. In the south, the North Atlantic craton (NAC) has a high-velocity keel exhibiting anisotropic stratification. Between the NAC and the cratonic lithosphere further north, the Proterozoic Nagssugtoqidian orogenic belt shows a distinct signature of reduced seismic velocity to ∼75 s period, but then appears to pinch out at depth. The northern Greenland lithosphere exhibits significant isotropic heterogeneity, with a distinct core of high velocities in the northwest (∼55-75 s period) giving way to a set of distinct east-west trending high-velocity belts at longer periods. At all periods sensitive to the lithospheric mantle in this region, anisotropic fast orientations are E-W, consistent with a north-south Precambrian assembly of the Greenland shield. In contrast to the NAC, there is no evidence of anisotropic stratification in the northern part of the cratonic keel. Based on both isotropic and anisotropic phase-velocity anomalies, we suggest that the Phanerozoic Caledonian and Ellesmerian-Franklinian fold belts are relatively thin-skinned features onshore Greenland, though the Caledonian belt may have a stronger signature off the east coast. At the longest periods, a prominent low-velocity anomaly initially centred on Iceland migrates northwards and spreads beneath central-eastern Greenland. Coupled with NW-SE trending anisotropy, this feature is interpreted as the effect of mantle flow radiating outward from the Iceland plume and interacting with the eroded Greenland lithosphere.

The role of continental heterogeneity on the evolution of continental plate margin topography at subduction zones

The nature of the overriding plate plays a major role for subduction zone processes. In particular, the highly heterogeneous continental lithosphere modulates intra-plate tectonics and the surface evolution of our planet. However, the role of continental heterogeneity is relatively under-explored for the dynamics of subduction models. We investigate the influence of rheological and density variations across the overriding plate on the evolution of continental lithosphere and slab dynamics in the upper mantle. We focus on the effects of variations in continental plate margin and keel properties on deformation, topographic signals, and basin formation. Our results show that the thickness, extent, and strength of the continental plate margin and subcontinental keel play a crucial role for the morphology and topography of the overriding plate, as well as the retreat of the subducting slab. We show that this lateral heterogeneity can directly influence the coupling between the subducting and overriding plate and determine the partitioning of plate velocities across the overriding plate. These findings suggest that back-arc extension and subsidence are not solely controlled by slab dynamics but are also influenced by continental plate margin and keel properties. Large extended back-arc regions, such as the Pannonian and Aegean basins, may result from fast slab rollback combined with a weak continental plate margin and a strong and extended continental keel. Narrow margins, like the Okinawa Trough in NE Japan, may indicate a comparatively stronger continental plate margin and weaker or smaller continental keel. Additionally, continental keel properties may affect the overall topography of the continental lithosphere, leading to uplift of the deformation front and the formation of intermontane basins.

Fertile upper mantle peridotite xenoliths indicate no wholesale destruction of cratonic root in East Asia

A major question in geodynamic evolution of the Earth is whether the continental keel evolved geochemically or were delaminated and replaced by a new mantle. Here we present direct evidence for the geochemical evolution of continental keel from lherzolite xenoliths preserved in the Cenozoic alkali basalts from Jeju Island, Korea. Our findings, using detailed petrography and geochemical methods, reveal distinct textures and compositional changes (represented by olivine with magnesium number 91–87, 86–61 respectively) of two stages of geochemical alteration. The first stage involved a parental kimberlite melt that penetrated during the Paleozoic, and the second was caused by alkali basalt during Cenozoic. Our findings suggest that the Archean depleted harzburgitic upper mantle partially evolved to lherzolitic through interaction with melts/fluids that originated from the deep mantle, implying no wholesale delamination of the upper mantle root of East Asian continents.

The Influence of Lithospheric Thickness Variations Beneath Australia on Seismic Anisotropy and Mantle Flow

AbstractRapid plate motion, alongside pronounced variations in age and thickness of the Australian continental lithosphere, makes it an excellent location to assess the relationship between seismic anisotropy and lithosphere‐asthenosphere dynamics. In this study, SKS and PKS shear‐wave splitting is conducted for 176 stations covering the transition from the South Australian Craton to eastern Phanerozoic Australia. Comparisons are made with models of lithospheric thickness as well as numerical simulations of mantle flow. Splitting results show uniform ENE‐WSW aligned fast directions over the Gawler Craton and broader South Australian Craton, similar to the orientation of crustal structures generated during an episode of NW‐SE directed compression and volcanism ∼1.6 billion years ago. We propose that heat from volcanism weakened the lithosphere, aiding widespread lithospheric deformation, which has since been preserved in the form of frozen‐in anisotropy. Conversely, over eastern Phanerozoic Australia, fast directions show strong alignment with the NNE absolute plate motion. Overall, our results suggest that when the lithosphere is thin (<125 km), lithospheric contributions are minimal and contributions from asthenospheric anisotropy dominate, reflecting shear of the underlying mantle by Australia's rapid plate motion above. Further insights from geodynamical simulations of the regional mantle flow field, which incorporate Australian and adjacent upper mantle structure, predict that asthenospheric material would be drawn in from the south and east toward the fast‐moving continental keel. Such a mechanism, alongside interactions between the flow field and lithospheric structure, provides a plausible explanation for smaller‐scale anomalous splitting patterns beneath eastern Australia that do not align with plate motion.

Build-up of multiple volatiles in Earth's continental keels: Implications for craton stability

Elevated contents of structurally-bound hydrogen in olivine, introduced to cratonic mantle during metasomatism, are widely believed to decrease the effective viscosity so that it is similar to the underlying asthenosphere (1018–1019 Pa⋅s) and susceptible to basal erosion. Nevertheless, large variations exist in H contents of mantle olivines from the Kaapvaal, Siberia, Slave, Tanzania and Wyoming cratons (0 to 320 ppmw) and the role of water as the main driver of rheological weakening is contentious. While recent experimental studies have shown that olivine has the capacity to host large quantities of fluorine, as well as hydrogen, the magnitude of this has not yet been established for cratonic mantle.Our new dataset for Kaapvaal craton peridotites shows that the style of metasomatism influences the addition of H2O and F to cratonic mantle. Silicic fluids derived from subducted slabs, and responsible for the pervasive orthopyroxene enrichment observed in the Kaapvaal (and many of the world's other cratons), deliver significant quantities of H2O but lesser amounts of F, whereas proto-kimberlite melts transport high quantities of both H2O and F. Kaapvaal mantle olivines are major hosts of both H2O (up to 105 ppmw) and F (up to 180 ppm), and we propose that a major increases in the bulk H2O (∼30%) and F (∼65%) of the Kaapvaal craton occurred over a short (20 Ma) time interval between the two main pulses of Cretaceous kimberlite emplacement in southern Africa.By combining the thermal structure of individual cratons with corresponding H2O data for olivine, we show that the effective viscosity contrast at the base of the mechanical boundary layer and asthenosphere below the Kaapvaal, Slave and Siberia cratons varies from 2 to 14. Some of the lowest effective viscosities (3.5×1017 Pa⋅s) occur in H2O-rich olivine in Siberia sheared peridotites from the base of the craton and are consistent with highly-localised metasomatism. More viscous peridotites (up to 1.5×1021 Pa⋅s) were entrained from the thermal boundary layer beneath the Kaapvaal and Tanzania cratons but residence times in this region are short due to convective overturn (<100 Ma) and this dehydrated mantle would offer only temporary protection to cratonic keels. Our viscosity estimates are based on the H content of olivine and, while the additional effect of structurally bound F in olivine is currently uncertain, we anticipate that it will have a similar effect on mantle rheology to H, so that the base of the mechanical boundary layer beneath cratons at the time and location of kimberlite generation is close to the tipping point for instability.The rapid build-up of volatiles associated with pervasive kimberlite activity may have been the catalyst for lithospheric thinning on the southern margin of the Kaapvaal craton, but more localised pulses of kimberlite activity occurred over longer time intervals in the Siberia and Slave cratons and so had a less profound effect on their stability. A prolonged and widespread subduction-flux of H2O to the Wyoming and N. China cratons, and subsequent sub-solidus partitioning in olivine, may have been the driver for the rheological weakening that ultimately led to delamination.

Archean cratonic mantle recycled at a mid-ocean ridge.

Basalts and mantle peridotites of mid-ocean ridges are thought to sample Earth’s upper mantle. Osmium isotopes of abyssal peridotites uniquely preserve melt extraction events throughout Earth history, but existing records only indicate ages up to ~2 billion years (Ga) ago. Thus, the memory of the suspected large volumes of mantle lithosphere that existed in Archean time (>2.5 Ga) has apparently been lost somehow. We report abyssal peridotites with melt-depletion ages up to 2.8 Ga, documented by extremely unradiogenic 187Os/188Os ratios (to as low as 0.1095) and refractory major elements that compositionally resemble the deep keels of Archean cratons. These oceanic rocks were thus derived from the once-extensive Archean continental keels that have been dislodged and recycled back into the mantle, the feasibility of which we confirm with numerical modeling. This unexpected connection between young oceanic and ancient continental lithosphere indicates an underappreciated degree of compositional recycling over time.

Mantle Structure and Flow Across the Continent‐Ocean Transition of the Eastern North American Margin: Anisotropic S‐Wave Tomography

AbstractLittle has been seismically imaged through the lithosphere and mantle at rifted margins across the continent‐ocean transition. A 2014–2015 community seismic experiment deployed broadband seismic instruments across the shoreline of the eastern North American rifted margin. Previous shear‐wave splitting along the margin shows several perplexing patterns of anisotropy, and by proxy, mantle flow. Neither margin parallel offshore fast azimuths nor null splitting on the continental coast obviously accord with absolute plate motion, paleo‐spreading, or rift‐induced anisotropy. Splitting measurements, however, offer no depth constraints on anisotropy. Additionally, mantle structure has not yet been imaged in detail across the continent‐ocean transition. We used teleseismic S, SKS, SKKS, and PKS splitting and differential travel times recorded on ocean‐bottom seismometers, regional seismic networks, and EarthScope Transportable Array stations to conduct joint isotropic/anisotropic tomography across the margin. The velocity model reveals a transition from fast, thick, continental keel to low velocity, thinned lithosphere eastward. Imaged short wavelength velocity anomalies can be largely explained by edge‐driven convection or shear‐driven upwelling. We also find that layered anisotropy is prevalent across the margin. The anisotropic fast polarization is parallel to the margin within the asthenosphere. This suggests margin parallel flow beneath the plate. The lower oceanic lithosphere preserves paleo‐spreading‐parallel anisotropy, while the continental lithosphere has complex anisotropy reflecting several Wilson cycles. These results demonstrate the complex and active nature of a margin which is traditionally considered tectonically inactive.

Seismic anisotropy in southern Costa Rica confirms upper mantle flow from the Pacific to the Caribbean

AbstractSurrounded by subducting slabs and continental keels, the upper mantle of the Pacific is largely prevented from mixing with surrounding areas. One possible outlet is beneath the southern part of the Central American isthmus, where regional observations of seismic anisotropy, temporal changes in isotopic composition of volcanic eruptions, and considerations of dynamic topography all suggest upper mantle flow from the Pacific to the Caribbean. We derive new constraints on the nature of seismic anisotropy in the upper mantle of southern Costa Rica from observations of birefringence in teleseismic shear waves. Fast and slow components separate by ∼1 s, with faster waves polarized along the 40°–50° (northeast) direction, near-orthogonally to the Central American convergent margin. Our results are consistent with upper mantle flow from the Pacific to the Caribbean and require an opening in the lithosphere subducting under the region.

Asthenospheric flow beneath the Carpathian-Pannonian region: Constraints from shear wave splitting analysis

Systematic shear wave splitting (SWS) study was conducted to illuminate the mantle deformation and dynamics in the Carpathian-Pannonian region (CPR) where its tectonic mechanism is still enigmatic and actively debated. We have obtained a total of 1170 SWS measurements using seismic data recorded by 59 broadband seismic stations. The resulting SWS measurements mostly possess NW-SE fast orientations and are mostly oblique or even orthogonal to the trends of major geological features and the absolute plate motions. Spatial coherency of the splitting parameters reveals that the center of the observed anisotropy is most likely originated from the asthenosphere at the depth of about 250 km. Thus, the uniformly NW-SE fast orientations is more likely to represent an asthenospheric flow, which has partially or completely overprinted the anisotropic fabrics developed during the formation of the Pannonian basin. Such an asthenospheric flow is regionally deflected along the edges of continental keels beneath the South Carpathians and the Vrancea zone where complex anisotropy is well-determined and range-parallel fast orientations are revealed based on two-layered fitting of the SWS measurements. Our results combined with previous SWS measurements suggest that the revealed NW-SE asthenospheric flow is actually a key component of the toroidal flow induced by the Adriatic subduction, which dominantly promotes the current upper mantle deformations beneath the CPR and its adjacent regions.

A new approach to computing steady-state geotherms: The marginal stability condition

A necessary physical condition for steady-state mantle convection is the marginal stability of convective boundary layer (CBL) accommodating the transition from conductive lithosphere to convective mantle. We incorporate the marginal stability condition (MSC) of the CBL into the lithosphere thermal modeling using it instead of the heat flow boundary condition specified on the surface. For the oceanic region, the MSC-based approach allows to calculate rather than postulate the thickness of the oceanic lithosphere beneath old oceanic crust areas. The model allows to estimate the potential temperature and to predict the depth at which the suboceanic CBL base occurs. The latter agrees well with the seismologically observed lithosphere-asthenosphere boundary. In the continental region, the CBL is immediately adjacent to the base of the chemical boundary layer comprising a crust and a melt-depleted continental keel. A deep segment of the MSC-based continental geotherm is almost independent of the uncertainty of the crustal heat production in the sense that two geotherms corresponding to the same lithosphere thickness and potential temperature but different crustal heat production converge at depth. Besides, the solution may be additionally adjusted so that the calculated surface heat flow matches observations without affecting the properties of the deeper geotherm segment. The model predicts quantitative relations between the chemical boundary layer thickness, the potential temperature of convecting mantle, and the lithospheric geotherm. The predictions correlate with the lithospheric geotherms documented using the kimberlite xenolith/xenocryst thermobarometry.

The longevity of Archean mantle residues in the convecting upper mantle and their role in young continent formation

The role played by ancient melt-depleted lithospheric mantle in preserving continental crust through time is critical in understanding how continents are built, disrupted and recycled. While it has become clear that much of the extant Archean crust is underpinned by Archean mantle roots, reports of Proterozoic melt depletion ages for peridotites erupted through Phanerozoic terranes raise the possibility that ancient buoyant lithospheric mantle acts as a “life-raft” for much of the Earth's continental crust. Here we report the largest crust–lithospheric mantle age decoupling (∼2.4 Ga) so far observed on Earth and examine the potential cause for such extreme age decoupling.The Phanerozoic (<300 Ma) continental crust of West Otago, New Zealand, is intruded by Cenozoic diatremes that have erupted cratonic mantle-like highly depleted harzburgites and dunites. These peridotites have rhenium depletion Os model ages that vary from 0.5 to 2.7 Ga, firmly establishing the record of an Archean depletion event. However, the vast range in depletion ages does not correlate with melt depletion or metasomatic tracer indices, providing little support for the presence of a significant volume of ancient mantle root beneath this region. Instead, the chemical and isotopic data are best explained by mixing of relict components of Archean depleted peridotitic mantle residues that have cycled through the asthenosphere over Ga timescales along with more fertile convecting mantle. Extensive melt depletion associated with the “docking” of these melt residues beneath the young continental crust of the Zealandia continent explains the decoupled age relationship that we observe today. Hence, the newly formed lithospheric root incorporates a mixture of ancient and modern mantle derived from the convecting mantle, cooled and accreted in recent times. We argue that in this case, the ancient components played no earlier role in continent stabilization, but their highly depleted nature along with that of their younger counterparts now represents a highly viscous, stable continental keel. This model could account for the large spectrum of ages observed in fertile to moderately depleted peridotites sampled from lithospheric mantle beneath SE Australia, W Antarctica and other locations in Zealandia, as well as the oceanic mantle. Our data confirm the longevity and dispersal of ancient depleted mantle domains in the convecting mantle and their re-appearance beneath young continents.

Lithospheric structure beneath Indochina block from Rayleigh wave phase velocity tomography

The Indochina block is important to our understanding of the extrusion model as a consequence of the Indo-Eurasia collision. The lithospheric structure of this block, however, remains obscured due to a lack of sufficient instrumentation for high resolution seismic imaging. We present a shear velocity model derived from Rayleigh wave phase velocity tomography using data from recently deployed seismic networks in this region. Our inversion results for lithospheric structure show strong correlations with tectonic history in this block. A prominent slow-velocity anomaly (5?per?cent) is observed in northern Indochina along the Ailao Shan-Red River (ASRR) shear zone including Chuxiong basin, Lanping-Simao fold belt and Thailand rift basin, which has seen extensive deformation events since Eocene. The Khorat Plateau basin is characterized by thick continental keel type lithosphere, consistent with palaeomagnetic and geological observations indicating this basin has experienced much less deformation than the surrounding regions. Additionally, our inversion imaged a sharp, lithospheric-scale velocity contrast across the southeastern segment of ASRR, indicative of a thin and thus relatively weak lithosphere southwest of Red River Fault. The thin lithosphere, low asthenospheric seismic velocities we observe and the average crustal thicknesses in the region suggest that the topography high is dynamically supported by upwelling asthenosphere rather than thickening of the crust/lithosphere. Based on the occurrence of Palaeogene volcanism and its timing, we prefer an explanation of thinning of the lithosphere and allowing a throughgoing fault rather than emplacement of a thin terrane to explain the thin lithosphere. Therefore, the anomalously thin lithosphere between Khorat Plateau and the ASRR in conjunction with other geological observations is generally consistent with the extrusion model for Indochina, which requires localization of lithospheric deformation around tectonic blocks.

History and dynamics of net rotation of the mantle and lithosphere

The net rotation of Earth's lithosphere with respect to the underlying mantle is the longest-wavelength component of toroidal flow in the mantle and is sensitive to both mantle buoyancy structure and lateral viscosity variations. The lithospheric net rotation in the geologic past implied by plate reconstructions using a hotspot reference frame for the past 100 Myr is up to five times greater than the present-day rate of lithospheric net rotation. We explore the role of lateral viscosity variations associated with subcontinental keels in producing the lithospheric net rotation for the geologic past and find that the introduction of subcontinental keels improves the agreement between modeled net rotation and the net rotation present in the plate reconstructions for the past 25 Myr. However, our models with continental keels produce at most of differential rotation between the lithosphere and lower mantle for present-day, and explaining the most rapid rates of lithospheric net rotation during the Cretaceous and Paleogene remains challenging. This suggests the need for either an additional mechanism for generating lithospheric net rotation, or an adjustment to the absolute mantle reference frame relative to which plate motions are specified.

Structure beneath the Alboran from geodynamic flow models and seismic anisotropy

Upper mantle heterogeneity beneath the Alboran Sea (western Mediterranean) as inferred from seismology has been associated with a range of subduction and lithospheric delamination scenarios. However, better constraints on the deep dynamics of the region are needed to determine the cause and consequence of complex surface tectonics. Here, we use an improved set of shear wave splitting observations and a suite of mantle flow models to test a range of suggested structures. We find that the observed seismic anisotropy is best reproduced by mantle flow models that include a continuous, deeply extending slab beneath the Alboran which elongates along the Iberian margin from Granada to Gibraltar and curves southward toward the High Atlas. Other models with detached slabs, slabs with spatial gaps, or drip‐like features produce results inconsistent with the splitting observations. SW‐directed shear flow, when combined with sublithospheric deflection in response to a dense sinker, generates NNW‐splitting orientations most similar to the patterns observed along Gibraltar. Slab viscosities of ∼250 times that of the upper mantle are preferred because they provide a balance between the poloidal flow induced by any sinker and toroidal flow induced by stiff slabs. The best match to anisotropy across the Atlas is a model with a stiff continental keel in northwestern Africa which deflects flow northward. Our results show that quantitative predictions of seismic anisotropy are useful in distinguishing the spatial and depth extent of regional density structures which may otherwise be ambiguous.

Sub-slab seismic anisotropy and mantle flow beneath the Caribbean and Scotia subduction zones: Effects of slab morphology and kinematics

Subduction systems are vitally important to plate tectonics and mantle convection, but questions remain about many aspects of subduction dynamics, particularly the nature of sub-slab mantle flow. Observations of seismic anisotropy can shed light on the pattern of mantle flow in subduction systems, but major uncertainties remain regarding the interpretation of sub-slab anisotropy. Here, we present measurements of shear wave splitting due to anisotropy in the sub-slab mantle beneath the Caribbean and Scotia subduction zones. These subduction systems are morphologically similar, with high arc curvature and short arc length, but there are differences between them in terms of their kinematic and tectonic settings. We apply the source-side shear wave splitting technique to direct teleseismic S phases originating from slab earthquakes. We restrict our analysis to seismic stations at which we have examined the receiver-side splitting in detail to ensure accurate corrections for anisotropy in the upper mantle beneath the station. We observe a well-defined pattern of trench-parallel fast directions (ϕ) in the northern half of the Caribbean subduction zone, with a transition to dominantly trench-perpendicular ϕ at the southern end. There is more scatter in the measurements for Scotia, but we observe generally trench-parallel ϕ in the northern portion of the subduction system, with a mix of trench-parallel, -perpendicular, and -oblique fast directions to the south. Our preferred interpretation of these splitting patterns is that they reflect 3-D return flow of the sub-slab mantle due to trench rollback. In both systems, we infer that sub-slab flow is being driven from south to north. Beneath Scotia, this is likely driven by differential migration of the Scotia trench along strike. Beneath the Caribbean, we hypothesize that mantle flow around the southern edge of the slab is inhibited by the presence of the South American continental keel, enabling trench-perpendicular stretching in the sub-slab mantle and forcing mantle flow to escape to the north. The similarities and differences we observe between the two systems yield insight into the relative contributions of slab morphology and plate kinematics in controlling mantle flow beneath subducting slabs.

The role of slabs and oceanic plate geometry in the net rotation of the lithosphere, trench motions, and slab return flow

Absolute plate motion models with respect to a deep mantle reference frame (e.g., hot spots) typically contain some net rotation (NR) of the lithosphere. Global mantle flow models for the present‐day plate setting reproduce similarly oriented NRs but with amplitudes significantly smaller than those found in some high NR Pacific hot spot reference frames. It is therefore important to understand the mechanisms of NR excitation, which we attempt here with two‐dimensional cylindrical models of an idealized Pacific domain. We study the influence of slab properties, oceanic ridge position, continental keels, and a weak asthenospheric layer on NR and trench migration. Fast slab return flow develops in models with stiff slabs and moderate slab dips. Rapid NRs, comparable to the high NR Pacific hot spot reference frames, are primarily induced by asymmetric slab dips, in particular a shallow slab beneath South America and a steep slab in the western Pacific. A scaling relationship links the amplitude of NR to plate size, slab dip angle, and slab viscosity. Asymmetric ridge positions also promote NR through asymmetric plate sizes. Continental keels have less impact, in contrast to what has been found in earlier global studies. Several models yield unidirectional Pacific trench motions, such as slab advance in the western Pacific and, simultaneously, slab retreat in the eastern Pacific. Our model provides a physical explanation for NR generation in the present‐day Pacific setting and hints at mechanisms for the temporal evolution of the basin.

Constraints on upper mantle viscosity from the flow‐induced pressure gradient across the Australian continental keel

The thickness of continental lithosphere varies considerably from tectonically active to cratonic regions, where it can be as thick as 250–300 km. Embedded in the upper mantle like a ship, when driven to move by a velocity imposed at the surface, a continental keel is expected to induce a pressure gradient in the mantle. We hypothesize that the viscosity of the asthenosphere or the shear coupling between lower lithosphere and asthenosphere should control this pressure effect and thus the resulting dynamic topography. We perform three‐dimensional finite element calculations to examine the effects of forcing a continental keel by an imposed surface velocity, with the Australian region as a case study. When the upper mantle is strong but still weaker than the lower mantle, positive dynamic topography is created around the leading edge, and negative dynamic topography is created around the trailing edge of the keel, which is measurable by positive and negative geoid anomalies, respectively. For a weak upper mantle the effect is much reduced. We analyze geoidal and gravity anomalies in the Australian region by spatiospectral localization using Slepian functions. The method allows us to remove a best fit estimate of the geographically localized low spherical harmonic degree contributions. Regional geoid anomalies thus filtered are on the order of ±10 m across the Australian continent, with a spatial pattern similar to that predicted by the models. The comparison of modeled and observed geoid anomalies places constraints on mantle viscosity structure. Models with a two‐layer mantle cannot sufficiently constrain the ratio of viscosity between the upper and lower mantle. The addition of a third, weak, upper mantle layer, an asthenosphere, amplifies the effects of keels. Our three‐layer models, with lower mantle viscosity of 3 × 1022 Pa s, suggest that the upper mantle (asthenosphere) is 300 times weaker than the lower mantle, while the transition zone (400–670 km depths) has a viscosity varying between 1021 and 1022 Pa s.

A translithospheric suture in the vanished 1-Ga lithospheric root of South India: Evidence from contrasting lithosphere sections in the Dharwar Craton

Garnet xenocrysts and eclogite xenoliths from 15 kimberlites (1.0–1.1 Ga in age) have been used to map the composition and structure of the lithospheric mantle along an 80-km traverse across the eastern margin of the Closepet Granite in Andhra Pradesh. The SCLM at the SW end of the traverse is more depleted (abundant harzburgites, mean X Mg of olivine ≈ 93.5, mean whole-rock Al 2O 3 ≈ 1.5%) than that at the NE end (fewer harzburgites, X Mg ≈ 92, Al 2O 3 > 2%). The depleted layer is ca 195 km thick in the SW, and ca 170 km in the NE, though geotherms are similarly low (ca 35–37 mW/m 2). The middle of the traverse is underlain by a strongly refertilised SCLM with a higher geotherm (ca 40 mW/m 2) and extensive evidence of metasomatism. At the SW end of the traverse, abundant eclogites are tightly concentrated in a layer from 175 to 190 km depth, coinciding with a zone of melt-related metasomatism. In the central part, eclogites are distributed through the highly metasomatised section from 90 to 160 km depth. These data suggest that the kimberlites at either end of the traverse sampled two distinct lithospheric blocks, perhaps coinciding with the Eastern and Western Dharwar Cratons. The zone of refertilised SCLM between them is interpreted as the cratonic suture, metasomatised by mafic melts (now eclogites). If this suture dips 70–80° to the east, its surface outcrop lies within the Closepet Granite. The “tilt” of the proposed cratonic suture may reflect overthrusting of the Eastern Dharwar Craton crust up to 100 km to the west. Recent geophysical data (seismic, MT) suggest that the depleted lithospheric root beneath the Dharwar Craton that was sampled by the 1.1 Ga kimberlites is no longer present. The removal or major modification of this root could have occurred during the breakup of Gondwanaland, and may help to explain India's rapid northward drift. India thus joins the North China Craton as an example of the destruction of an Archean continental keel.

A fluid factory in solid Earth

Global material circulation in our planet from the surface to the bottom of the mantle is controlled by a combination of plate, plume, and “anti–plate” tectonics, where fluids and melts play an active role. Whereas crustal fluids are dominated by CO 2 and H 2 O, with subordinate CH 4 and N 2 , the volatiles in the lower mantle are speculated to be dominantly CO 2 . The process of Archean subduction aided in the sequestration of CO 2 from the ocean-atmosphere system, with the ultimate probable destination being the mantle. When a rising “superplume” hits the tectosphere, the predicted carbonated continental keel would become enriched in CO 2 . During the Phanerozoic, the carbonated upper mantle was drastically reduced in size, as speculated from the scarcity of dry, “ultrahot” orogens. Free fluid circulation within Earth is present only in restricted zones, mostly along the plate boundaries and intracontinental rifts, and particularly along subduction zones, where the fluid is mostly water dominated. The propagating water front in the deep mantle in modern Earth may correspond to the apparent increase in pressure through geologic time, which might be one of the reasons for a general lack of ultrahigh-pressure metamorphic belts in the Archean and their prevalence in the Phanerozoic orogenic belts.

Azimuthal seismic anisotropy constrains net rotation of the lithosphere

Net rotation (NR) of the lithosphere is found in hot spot reference frames, important for tectonics and plume models, but difficult to constrain. Using mantle flow and crystallographic texture modeling, I show that NRs lead to mantle shearing which is recorded in azimuthal seismic anisotropy. The NR amplitude in some hot spot models is so large that it degrades the model fit to anisotropy significantly. Smaller NRs, such as predicted by flow models with stiff continental keels, are consistent with seismology, however.