Upper-mantle Convection Research Articles

Small-scale upper mantle convection and orogeny of Tianshan Mountains in China

In this study, from the travel time data recorded in the Tianshan passive seismic array experiment, we present the P-wave velocity structure of the upper mantle down to 660 km along the Kuqa-Kuitun profile in terms of seismic tomography technique. Based on the P-wave velocity model, we derive the corresponding 2D upper mantle density model. The 2D small-scale convection of the upper mantle underneath the Tianshan Mountains in China driven by the density anomalies is simulated using the hybrid finite element method combining with the marker-in-cell technique. The main features of the upper mantle convection and the reciprocation between the convection and mountain building are investigated. The results manifest that (1) in the upper mantle underneath the Junggar basin and North Tianshan exists a counterclockwise convection, which scale is ∼ 500 km; (2) underneath the Tarim basin and South Tianshan exists a clockwise northward convection, which is relatively weak; (3) the convective velocity at the top of the upper mantle underneath the Tianshan Mountains in China should not be less than 20 mm/a, while considering the dependent of convective velocity on the viscosity; (4) the northward extrusion of the Tarim block plays a key role in the Cenozoic Tianshan mountain building and the present-day tectonic deformation of the Tianshan range is related closely to the upper mantle convection; and (5) the northward subduction of the Tarim block does not influence obviously the upper mantle convection.

Small-scale convection in the upper mantle beneath the Chinese Tian Shan Mountains

Small-scale convection of the upper mantle beneath the Chinese Tian Shan (Tien Shan) is investigated in terms of numerical modeling. The finite element method combined with the marker-in-cell technique is used to describe the flow of the heterogeneous upper mantle. The density model is derived from the P-wave velocity structure of the crust and upper mantle along the Kuche–Kuitun profile across the Chinese Tian Shan, which is obtained using the seismic travel time tomography technique. Our computational results reveal the southward-counterclockwise and northward-clockwise upper mantle convective cells underneath the Junggar-north Tian Shan and Tarim-south Tian Shan, respectively. Our results also show the convective scale reaches to ∼500 km and the convective speed at the top of the upper mantle should not be less than 20 mm/year for a normal viscosity model. The northward extrusion of the Tarim block plays a key role in the Tian Shan mountain building since the Cenozoic period, but it nearly does not influence the upper mantle convection. The present-day tectonic deformation in the Chinese Tian Shan is related to the small-scale convection of the upper mantle.

Regional uplift associated with continental large igneous provinces: The roles of mantle plumes and the lithosphere

The timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes. Here we review geological indicators of surface uplift associated with five continent-based magmatic provinces: Emeishan Traps (260 million years ago: Ma), Siberian Traps (251 Ma), Deccan Traps (65 Ma), North Atlantic (Phase 1, 61 Ma and Phase 2, 55 Ma), and Yellowstone (16 Ma to recent). All five magmatic provinces were associated with surface uplift. Surface uplift can be measured directly from sedimentary indicators of sea-level in the North Atlantic and from geomorpholocial indicators of relative uplift and tilting in Yellowstone. In the other provinces, surface uplift is inferred from the record of erosion. In the Deccan, North Atlantic and Emeishan provinces, transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma. Transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation. Observations of the spatial and temporal relationships between surface uplift, rifting and magmatism are also important in constraining models of LIP formation. Onset of surface uplift preceded magmatism in all five of the provinces. Biostratigraphic constraints on timing of uplift and erosion are best for the North Atlantic and Emeishan Provinces, where the time interval between significant uplift and first magmatism is less than 1 million years and 2.5 million years respectively. Rifting post-dates the earliest magmatism in the case of the North Atlantic Phase 1 and possibly in the case of Siberia. The relative age of onset of offshore rifting is not well constrained for the Deccan and the importance of rifting in controlling magmatism is disputed in the Emeishan and Yellowstone Provinces. In these examples, rifting is not a requirement for onset of LIP magmatism but melting rates are significantly increased when rifting occurs. Models that attempt to explain emplacement of these five LIPs without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift, rifting and magmatism. For example, small-scale convection related to craton or rift boundaries (edge-driven convection) cannot easily explain widespread (1000 km scale) transient surface uplift (Emeishan, Deccan, North Atlantic), and upper mantle convection initiated by differential incubation beneath cratons (the hotcell model) is at odds with rapid onset of surface uplift (Emeishan, North Atlantic). The start-up plume concept is still the most parsimonious way of explaining the observations presented here. However, observations of surface uplift cannot directly constrain the depth of origin of the hot mantle in a plume head. The short time interval between onset of transient surface uplift and magmatism in the North Atlantic and Emeishan means that the associated starting plume heads were probably not large (∼ 1000 km diameter) roughly spherical diapirs and are likely to have formed narrow (∼ 100 km radius) upwelling jets, with hot mantle then spreading rapidly outward within the asthenosphere. In cases where rifting post-dates magmatism (N Atlantic Phase 1) or where the degree of lithospheric extension may not have been great (Siberia), a secondary mechanism of lithospheric thinning, such as gravitational instability or delamination of the lower lithosphere, may be required to allow hot mantle to decompress sufficiently to explain the observed volume of magma with a shallow melting geochemical signature. Any such additional thinning mechanisms are probably a direct consequence of plume head emplacement.

Regional magnetic anomalies, crustal strength, and the location of the northern Cordilleran fold-and-thrust belt

The northern Cordilleran fold-and-thrust belt in Canada and Alaska is at the boundary between the broad continental margin mobile belt and the stable North American craton. The fold-and-thrust belt is marked by several significant changes in geometry: cratonward extensions in the central Yukon Territory and northeastern Alaska are separated by marginward re-entrants. These geometric features of the Cordilleran mobile belt are controlled by relations between lithospheric strength and compressional tectonic forces developed along the continental margin. Regional magnetic anomalies indicate deep thermal and compositional characteristics that contribute to variations in crustal strength. Our detailed analysis of one such anomaly, the North Slope deep magnetic high, helps to explain the geometry of the fold-and-thrust front in northern Alaska. This large magnetic anomaly is inferred to reflect voluminous mafic magmatism in an old (Devonian?) extensional domain. The presence of massive amounts of mafic material in the lower crust implies geochemical depletion of the underlying upper mantle, which serves to strengthen the lithosphere against thermal erosion by upper mantle convection. We infer that deep-source magnetic highs are an important indicator of strong lower crust and upper mantle. This stronger lithosphere forms buttresses that play an important role in the structural development of the northern Cordilleran fold-and-thrust belt.

Upper mantle convection beneath northwest China and its adjacent region driven by density anomaly

We assume that the density anomalies, which are transformed from seismic tomography data, are corresponding to temperature distribution in a convective mantle. We take density anomalies as the driving force for mantle convection and solve the basic equation with given boundary conditions in a wave-number domain by using the FFT arithmetic. Using the physical model of upper mantle convection and the seismic tomography data supplied by XU et al, we calculated upper mantle convection beneath northwestern China and adjacent region. The flow patterns in the upper mantle show that there are upward and divergent flows in the basin regions, such as Tarim, Qaidam, Junggar and Kazakhstan, where the lithosphere is thin. There are downward and convergent flows in the mountain regions, such as Tianshan, Kunlun and Qilian, where the lithosphere is thick. In addition, because of the divergent flow under the Tarim Basin the upper mantle material in this region is driven southward to the north part of Tibetan Plateau and northward to Tianshan Mountain. Maybe, it is one of the reasons for the recent uplift of the Tianshan Mountain.

Imag(in)ing the continental lithosphere

This paper is primarily concerned with seismically imaging details in the mantle at an intermediate scale length between the large scales of regional and global tomography and the small scales of reflection profiles and outcrops. This range is roughly 0.1–1 km < a < 10–10 2 km, where a is the scale. We consider the implications of several models for mantle evolution in a convecting mantle, and possible scales present in the non-convecting tectosphere. Reflection seismic evidence shows that the structures preserved from continental accretion within and at the margins of the Archean cratons are subduction related, and we use subduction as an analog for scales left by past events. In modern orogenic belts we expect to find subduction structures, small scale upper mantle convection structures, and basalt extraction structures. We examine some of the scales that are likely formed by orogenic processes. We also examine the seismic velocity and density contrasts expected between various upper mantle constituents, including fertile upper mantle, depleted upper mantle, normal and eclogitized oceanic crust, and fertile mantle with and without partial melt. This leads directly to predicting the size of seismic signals that can be produced by specular conversion, and scattering from layers and objects with these contrasts. We introduce an imaging scheme that makes use of scattered waves in teleseismic receiver functions to make a depth migrated image of a pseudo-scattering coefficient. Image resolution is theoretically at least an order of magnitude better than traveltime tomography. We apply the imaging scheme to three data sets from 1) the Kaapvaal craton, 2) the Cheyenne Belt, a Paleoproterozoic suture between a protocontinent and an island arc, and 3) the Jemez Lineament, a series of aligned modern volcanic structures at the site of a Proterozoic suture zone. The Kaapvaal image, although not defining a unique base of the tectosphere, shows complicated “layered” events in the region defined as the base of the tectosphere in tomography images. The image of the transition zone discontinuities beneath the Kaapvaal craton is remarkable for clarity. The migrated receiver function image of the upper mantle beneath the Cheyenne belt is complicated, more so than the tomography image, and may indicate limitations in the receiver function imaging system. In contrast the Jemez Lineament image shows large-amplitude negative-polarity layered events beneath the Moho to depths of ∼120 km, that we interpret as melt-containing sills in the upper mantle. These sills presumably feed the Quaternary–Neogene regional basaltic volcanic field.

Episodic Cenozoic tectonism and the development of the NW European ‘passive’ continental margin

The North Atlantic margins are archetypally passive, yet they have experienced post-rift vertical movements of up to kilometre scale. The Cenozoic history of such movements along the NW European margin, from Ireland to mid-Norway, is examined by integrating published analyses of uplift and subsidence with higher resolution tectono-stratigraphic indicators of relative movements (including results from the STRATAGEM project). Three episodes of epeirogenic movement are identified, in the early, mid- and late Cenozoic, distinct from at least one phase of compressive tectonism. Two forms of epeirogenic movement are recognised, referred to as tilting (coeval subsidence and uplift, rotations <1° over distances of 100s of Kilometres) and sagging (strongly differential subsidence, rotations up to 4° over distances <100 km). Each epeirogenic episode involved relatively rapid (<10 Ma) km-scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity-bounded stratigraphic units. Early Cenozoic tilting (late Paleocene to early Eocene, c. 60–50 Ma) caused the basinward progradation of shelf-slope wedges from elongate uplifts along the inner continental margin and from offshore highs. Mid-Cenozoic sagging (late Eocene to early Oligocene, c. 35–25 Ma) ended wedge progradation and caused the onset of contourite deposition in deep-water basins. Late Cenozoic tilting (early Pliocene to present, <4±0.5 Ma) again caused the basinward progradation of shelf-slope wedges, from uplifts along the inner margin (including broad dome-like features) and from offshore highs. The early, mid- and late Cenozoic epeirogenic episodes coincided with Atlantic plate reorganisations, but the observed km-scale tectonic movements are too large to be accounted for as flexural deflections due to intra-plate stress variations. Mantle–lithosphere interactions are implied, but the succession of epeirogenic episodes, of differing form, are difficult to reconcile with the various syn-to post-rift mechanisms of permanent and/or transient movements proposed in the hypothetical context of a plume beneath Iceland. The epeirogenic movements can be explained as dynamic topographic responses to changing forms of small-scale convective flow in the upper mantle: tilting as coeval upwelling and downwelling above an edge-driven convection cell, sagging as a loss of dynamic support above a former upwelling. The inferred Cenozoic succession of epeirogenic tilting, sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening, a process that may also be the underlying cause of plate reorganisations. The postulated episodes of flow reorganisation in the NE Atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin, as well as on other Atlantic-type ‘passive’ margins.

The structure and the surface manifestation of mantle plumes in depth-dependent three-dimensional models

Thermal convection has been modeled using a 3D Cartesian model, in order to study the structure of mantle plumes and their surface manifestation. Hotspots can be regarded as the surface manifestation of mantle plumes. The main characteristic features of the hotspots are their volcanism, topographic, geoid and heat flow anomalies. The following characteristics of the hotspot-generator plumes are studied: their geometrical size, temperature distribution, the horizontal extent and maximum amplitude of the surface features: geoid, topographic and heat flow anomalies. The aim of the model calculations is to investigate the effect of depth-dependent viscosity and internal heating in case of scaling the non-dimensional result for upper and whole mantle convection. When upper mantle convection is supposed, the calculated amplitudes of the anomalies fit well to the observed values, but their lateral extent is too small. The extent of the anomalies suggests whole mantle plume. Taking into account the high viscosity of the lithosphere, the low viscosity of the asthenosphere and the D″ layer at the CMB and using internal heating helps to gain better fit to the observations. The most complex whole mantle model predicts quite realistic features, except the topographic height.

Upper mantle convection driving by density anomaly and a test model

A mathematic-physical convection model in upper mantle is established in this paper. This model is in a three dimensional frame. Assume that the density anomalies, which are mapped from the seismic tomography data, are responding to the temperature perturbation in the connective system. This model takes the density anomalies as the driving force for the mantle convection. We solve the basic equations with different boundary conditions in a domination of wave number by using the FFT arithmetic. In order to test the validity of our theory and method two simple models (higher-lower density body and break off slab) are employed to calculate their flow patters in this paper. Results show that our method can be directly used to investigate the upper mantle convection patterns and its related problems of lithospheric dynamics.

Neogene evolution of the Atlantic continental margin of NW Europe (Lofoten Islands to SW Ireland): anything but passive

A regional stratigraphic framework for the Neogene succession along and across the NW European margin is presented, based on a regional seismic and sample database. The stratigraphy provides constraints on the timing and nature of the mid- to late Cenozoic differential tectonic movements that have drivenmajor changes in sediment supply, oceanographic circulation and climate (culminating in continental glaciation). The overall context for Neogene deposition on the margin was established in the mid-Cenozoic, when rapid, km-scale differential subsidence (sagging) created the present-day deep-water basins. The Neogene is subdivided into lower (Miocene–lower Pliocene) and upper (lower Pliocene–Holocene) intervals. The lower Neogene contains evidence of early to mid-Miocene compressive tectonism, including inversion anticlines and multiple unconformities that record uplift and erosion of basin margins, as well as changes in deep-water currents. These movements culminated in a major expansion of contourite drifts in the mid-Miocene, argued to reflect enhanced deep-water exchange across the Wyville-Thomson Ridge Complex, via the Faroe Conduit. The distribution and amplitude of the intra-Miocene movements are consistent with deformation and basin margin flexure in response to enhanced intra-plate compressive stresses during a local plate reorganization (transfer of the Jan Mayen Ridge from Greenland to Europe). The upper Neogene records a seaward tilting (&lt;1°) of the margin from the early Pliocene, in which km-scale uplift and erosion was accompanied by increased offshore subsidence, resulting in a major seaward progradation of the shelf–slope wedge as well as deep-marine erosion during a reorganization of bottom current patterns. The large amplitude of tilting cannot be accounted for by intra-plate stress variations, but is consistent with a dynamic topographic response to upper mantle convection, in particular edge-driven flow beneath the continental margin. Sedimentary and oceanographic changes resulting from dynamic topographic responses to the evolution of upper mantle convective flow during ocean widening may be characteristic of the development of ‘passive’ continental margins.

Mantle transition zone topography and structure beneath the Yellowstone hotspot

The depths of the 410 and 660 km discontinuities beneath the Yellowstone hotspot are constrained using common conversion point imaging of P‐wave receiver functions. The mean depth of the 410 and 660 is 411 ± 1.5 km and 656 ± 1.6 km, with 36–40 km of peak to peak topography. This topography is spatially uncorrelated, providing no evidence for a lower mantle plume currently beneath the hotspot. The topography suggests that ±200°C thermal anomalies exist with respect to an average mantle adiabat. Two warmer than normal regions are found: at the 410 to the NNW of the hotspot and at the 660 to the NE. Colder temperatures exist at the 410 under central Wyoming. Upper mantle convection and/or intermittent heat and mass transfer across the 660 may be responsible for the uncorrelated topography. Three negative arrivals about the 410 and 660 are observed that display correct Pds moveout.

Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography

We present models for upper mantle P and S velocity structure beneath a southwestern United States transect extending from near the center of the Colorado Plateau across the Rio Grande rift to the Great Plains. The models were derived from travel times of compressional and shear seismic phases recorded by the La Ristra passive seismic array deployed from July 1999 to May 2001. Large variations in seismic velocity (up to 8% in S and 5% in P) are seen across the transect in the upper 200 km of the mantle. Seismically slow mantle underlies the Rio Grande rift and Jemez lineament and relatively slow mantle is seen beneath the Navajo volcanic field within the Colorado Plateau. The relative variations of P and S velocity imply that high temperatures are the primary cause of the slow mantle although a small amount of partial melting or hydration cannot be ruled out. Sharp boundaries in mantle seismic velocity are coincident with boundaries of Proterozoic structural trends implying that ancient lithospheric structure exerts a control on the tectonic and magmatic activity in the region. Weaker seismic variations are imaged below 200 km depth with three southeastward dipping structures in our images. Two of the structures have fast seismic anomalies, beneath the central Colorado Plateau and the Great Plains respectively, and the third has anomalously slow seismic wave speed. We interpret the western deep seismic anomaly to be foundering Farallon slab and the slow anomaly just to the east as upwelling mantle possibly associated with volatile release from the sinking Farallon slab. Beneath the Great Plains, there is also downwelling in the upper mantle. The combination of upwelling in the west and downwelling in the east are likely part of an upper mantle convection cell that may include entrained lithosphere from beneath the rift.

Abrupt axial variations along the slow to ultra-slow spreading centers of the northern North Fiji Basin (SW Pacific): Evidence for short wave heterogeneities in a back-arc mantle

This study presents results of surveys conducted along the slow to ultra-slow spreading axis of the Northern North Fiji Basin (NNFB), including the Hazel Holmes, Tripartite and South Pandora Ridges, and the newly discovered Futuna and North Cikobia spreading centers. Spreading segments along these axes display highly contrasted axial morphologies, ranging from a rift valley to a prominent axial high. In some places, abrupt inversions of topography are observed between neighboring segments. Detailed analyses of bathymetry and backscatter maps reveal that axial highs are spotted with numerous coalescent volcanoes forming features ranging from irregular terrains to well-organized ridges. The volcanic edifices are distributed over a wide neovolcanic zone, which corresponds to the axial relief, suggesting on important contribution of volcanism to the relief construction. Comparisons between various ridge-shaped segments reveal that axial volcano-tectonic patterns are directly related to the local magma production and delivery, in a context of tectonic extension related to plate divergence, and suggest that coalescent volcanoes are fed from multiples short-lived and unconnected magma lenses. In the competition between horizontal and vertical accretion of oceanic crust, the spreading centers of the NNFB represent a special case where lava production is locally high enough and spreading rate is low enough to allow prominent axial highs to develop. The along axis morphologic variability is related to intermittent volcanic activity that may result from rapid temporal and spatial variations in the distribution of upper mantle convection cells below accretion centers, superimposed on the regional thermal anomaly located under the whole basin.

Composition and evolution of submarine volcanic rocks from the central and western Canary Islands

Submarine volcanic rocks dredged during RV Meteor cruise M43-1 comprise alkali basalts, basanites, nephelinites and their differentiates representing both basement-shield and young post-shield volcanics of Gran Canaria, Tenerife, La Palma and El Hierro. The primitive lavas vary widely in trace element composition (e.g., Zr/Y=6.6–11.7, (La/Sm)N=2.3–5.4, and Ba/Yb=71–311), and they are characterized by steep, rare-earth element patterns with mean (La/Yb)N=16, and by pronounced, positive primitive mantle-normalized Nb and Ta and negative K anomalies similar to HIMU-type basalts. Rocks from the submarine flanks west and north of Gran Canaria are isotopically and geochemically identical to rocks of the subaerial Miocene shield stage, but they are distinct from rocks of the post-shield stages (Zr/Nb=6.3–8.9, 87Sr/86Sr=0.70327–0.70332, 143Nd/144Nd=0.51289–0.51293, 206Pb/204Pb=19.55–19.88). Most rocks dredged from the submarine flanks of Tenerife are isotopically and geochemically similar to rocks of the adjacent subaerial shield remnants, but a few resemble rocks of the subaerial post-shield stages (total range in Zr/Nb=4.6–6.1, 87Sr/86Sr=0.70300–0.70329, 143Nd/144Nd=0.51281–0.51292, 206Pb/204Pb=19.51–19.96). Rocks from the southern submarine ridge of La Palma cover the entire compositional range of the subaerial rocks of that ridge. Additionally, they comprise a high Zr/Nb group which resembles rocks of the ca. 1-Ma-old Taburiente shield of northern La Palma (total range in Zr/Nb=3.0–6.4, 87Sr/86Sr=0.70297–0.70314, 143Nd/144Nd=0.51288–0.51296, 206Pb/204Pb=19.21–19.79). Rocks from the southern submarine ridge of El Hierro compositionally resemble subaerial rocks of the island (Zr/Nb=4.1–6.2, 87Sr/86Sr=0.70296–0.70314, 143Nd/144Nd=0.51291–0.51297, 206Pb/204Pb=19.25–19.91). The degree of melting in the subcanarian mantle is interpreted to decrease from east to west across the archipelago whereas the proportion of depleted mantle component in the melting anomaly increases, as illustrated by Sr, Nd and Pb isotopes. The isotopic characteristics of the mantle source beneath the Canary Islands represents a mixture of HIMU, DMM and EM I. The overall isotopic signature is intermediate between that of Madeira to the north, which trends towards more depleted compositions, and that of the Cape Verde Islands to the south which shows a pronounced trend towards enriched mantle compositions. A clear trend towards the EM II component is only evident in more evolved rocks dredged from a seamount between Tenerife and Gran Canaria, some of which contain terrigenous sedimentary xenoliths. We propose a genetic model which relates similar mantle source signatures of volcanic archipelagos off West Africa to a common, large-scale lower mantle upwelling which, according to geophysical data, becomes more diffuse in the upper mantle. Narrow plumes or blobs feeding the volcanic centers along the passive margin may rise from this thermal anomaly due to upwelling in small, continent-parallel upper-mantle convection cells.

Numerical Simulation of the Uplift Process of the Qinghai‐Xizang Plateau under the Effect of Denudation and Mantle Convection

AbstractThe continuity equation that controls the continental deformation in the England and Mckenzie's thin viscous sheet model is modified. We directly introduce the effect of denudation during the uplift process of the Qinghai‐Xizang Plateau into the continuity equation, and the effect of removal caused by the small‐scale mantle convection beneath this region at the bottom of the thickening mantle lithosphere is considered during late uplift period of the plateau. We also use the method of finite difference to directly simulate the uplift process of the Qinghai‐Xizang Plateau. The results of the numerical simulation are consistent with the observed data, and reveal that the process of the plateau's uplift is multistage and unstable. The results also indicate that the removal caused by the small‐scale upper mantle convection at the bottom of the thickening mantle lithosphere may be the main mechanism of the rapid uplift of the plateau in the last 8 Ma.

Seismic anisotropy: tracing plate dynamics in the mantle.

Elastic anisotropy is present where the speed of a seismic wave depends on its direction. In Earth's mantle, elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation. Earthquakes generate two seismic wave types: compressional (P) and shear (S) waves, whose coupling in anisotropic rocks leads to scattering, birefringence, and waves with hybrid polarizations. This varied behavior is helping geophysicists explore rock textures within Earth's mantle and crust, map present-day upper-mantle convection, and study the formation of lithospheric plates and the accretion of continents in Earth history.

On the state of sublithospheric upper mantle beneath a supercontinent

An assumption of isothermal and static asthenosphere as the normal state of sublithospheric mantle, as commonly employed in studies of terrestrial magmatism, may be physically implausible for a system cooled from above. The growth of an unstable thermal boundary layer at the base of the lithosphere can lead to small-scale convection, so asthenosphere is expected to be usually convecting. A first-order estimate based on the energetics of convection suggests that mantle upwelling rate associated with such small-scale convection is on the order of 10 cm yr−1 for asthenospheric viscosity of 1018–1019 Pa s. To investigate the potential role of such sublithospheric convection in anomalous magmatism associated with rifting of the supercontinent Pangaea, a simple upper-mantle convection problem with thick cratonic lithosphere is considered through finite element modelling. Strong three dimensionality is exhibited in our solutions, and the planform of convection is largely affected by the bottom topography of continental lithosphere. Our model also suggests that differential cooling, imposed by a variation in lithospheric lid thickness, may lead to large-scale convection, which brings uncooled, ‘hot’ material from the base of thick lithosphere to shallow asthenosphere. The potential importance of such sublithospheric convection in magmatism during continental rifting is discussed.

Upper-mantle origin of the Yellowstone hotspot

Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than ∼200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis.

Geophysics. Top-down tectonics?

Geophysicists disagree whether Earth's mantle is a single convective cell or whether it is divided into convective layers. In his Perspective, Anderson reviews the evidence and concludes that the case for layered convection is strong. He also argues that plate tectonics may be driving upper mantle convection, rather than the other way around as traditionally believed. Sophisticated convection simulations will be needed to test this hypothesis.

Heat flow scaling for mantle convection below a conducting lid: Resolving seemingly inconsistent modeling results regarding continental heat flow

Numerical models of mantle convection below a conducting lid, meant to mimic a continent, have been used 1) to argue that heat flow variations in stable continental regions result principally from lithospheric thickness variations, and 2) to argue that the relationship between lithospheric thickness and continental heat flow is too weak to allow for this. We reconcile these results using a theoretical heat flow scaling which shows that the relationship between heat flow and lid thickness can take on two end‐member forms. As the ratio of lid thickness to convecting layer depth increases, the effects of lid thickness variations on heat flow move from weak to strong. Modeling studies that concluded that lithospheric thickness variations could lead to significant continental heat flow variations assumed a convecting layer depth appropriate to upper mantle convection and, thus, maximized the relative thickness of the continental lithosphere, i.e., the conducting lid. Studies that concluded that the relationship is weak assumed whole mantle convection which placed them on the flatter end of the heat flow versus relative lid thickness curve.