Warm Mantle Research Articles

Deep mantle forces and the uplift of the Colorado Plateau

We introduce a quantitative model of global mantle convection that reconstructs the detailed motion of a warm mantle upwelling over the last 30 Ma towards the interior of the southwestern USA from observed present‐day mantle heterogeneity. The onset and evolution of uplift in the central Basin and Range province and Colorado Plateau during this time is determined by tracking the topographic swell due to this mantle upwelling. We show that: (1) the extension and basaltic volcanism (post 25 Ma) in the central Basin and Range coincides with the arrival and eastward progression of this upwelling, and (2) dynamic uplift of the southern Colorado Plateau, totaling about 1 km, transpired in the last 20 Ma. Since 10 Ma, the center of uplift continued northeastward from the southwestern rim of the plateau consistent with a young Grand Canyon model and eastward sweep of magmatism in the western Colorado Plateau.

Reactivation of an Archean craton: Constraints from P‐ and S‐wave tomography in North China

The unusual reactivation of the North China Craton (NCC) challenges the classical views concerning the strength and stability of cratonic lithosphere. By using teleseismic body‐waves recorded at 250 seismic stations, this paper presents high‐resolution North China Models of P‐ and S‐wave velocity based on finite‐frequency kernel tomography. Both P‐ and S‐wave velocity models reveal that: (1) an obvious N–S trending narrow low‐velocity region is located at the base of the lithosphere beneath the Central Block (CB) of the NCC, which extends to more than 500 km depth; (2) a region of high‐velocity extends to more than 250–300 km depth beneath the Western Block, in contrast to the much shallower high‐velocity zones beneath the CB and shallower high‐velocities beneath the Eastern Block. These features suggest that warm mantle material with a source at least as deep as the transition zone, possibly a mantle plume, may be responsible for the reactivation of the NCC. The Central Block may have behaved as a sublithospheric corridor for the warm mantle material due to its pre‐existing weakness.

The topography and morphology of low shields and associated landforms of plains volcanism in the Tharsis region of Mars

The morphology and topography of volcanic landforms provide critical information to the investigation of their tectonic setting and the physical characteristics (e.g., rheology) of their eruption products. Their investigation is also an important prerequisite for studies of comparative planetology, (e.g., the comparison between surface features of the Earth and other planetary bodies). Numerous small and low shield volcanoes on Mars and associated vents and lava flows have previously been compared to terrestrial plains-style volcanism, which is defined as being an intermediate style between flood basalts and the Hawai'ian shields. This study investigates the topography and morphology of Martian landforms associated with plains volcanism using MOLA, MOC, THEMIS, HRSC, and HiRISE data. Low shields and other landforms of plains volcanism on Mars display similarities with terrestrial basaltic volcanic fields, and we do not observe any features that do not have morphologic analogues on Earth. The most typical landforms of Martian plains-style volcanism are low shields, defined as volcanic shields with diameters of typically less than 50 km, heights of a few hundred meters only, and extremely shallow flank slopes of less than 0.5°. Other surface features related to plains-style volcanism on Mars are craters, fissure vents, cinder and spatter cones, lava flows (that are commonly associated with lava channels and tubes), lava inflation features, and volcanic rift zones. Our results reconfirm the Viking Orbiter-based conclusion that plains volcanism in the eastern Snake River Plains is perhaps the best terrestrial morphological analogue for these Martian surface features. Icelandic shields, distinct structures in Hawai'i, and other basaltic landforms also show similarities to Martian plains volcanism. Sinuous rilles, previously not described in association with plains volcanism on Mars, are interpreted as evidence for high eruption rates. The extremely shallow flank slopes of the low shields suggest the eruption of shield-building lavas with low viscosity, which might be the result of high eruption temperatures, high effusion rates, a low Si- and a high Mg-content along with a possible high Fe-content, or a combination of these factors. The spatial distribution of low shield clusters in Tharsis does not show any obvious association with large-scale tectonic features. Plains volcanism might represent a relatively recent type of volcanism on Mars, which is not related to mantle plumes but to a zone of partial melting in an anomalously warm mantle underneath a thickened crust (Schumacher and Breuer, Geophys. Res. Lett. 34, L14202, doi: 10.1029/2007GL030083, 2007).

Mantle transition zone structure beneath Kenya and Tanzania: more evidence for a deep-seated thermal upwelling in the mantle

SUMMARY Here we investigate the thermal structure of the mantle beneath the eastern Branch of the East African Rift system in Kenya and Tanzania. We focus on the structure of the mantle transition zone,asdelineatedbystackingofreceiverfunctions.Thetopofthetransitionzone(the410km discontinuity) displays distinctive topography, and is systematically depressed beneath the rift in Kenya and northern Tanzania and adjacent volcanic fields. This depression is indicative of a localized ! 350 C thermal anomaly. In contrast, the bottom of the transition zone (the 660km discontinuity) is everywhere depressed. This region-wide depression is best explained as a Ps conversion from the majorite‐perovskite transition of anomalously warm mantle. We interpret this structure of the transition zone as resulting from the ponding of a mantle plume (possibly the deep-mantle African Superplume) at the base of the transition zone, which then drives localized thermal upwellings that disrupt the top of the transition zone and extend to shallow mantle depths beneath the rift in Kenya and northern Tanzania.

The African upper mantle and its relationship to tectonics and surface geology

This paper focuses on the upper-mantle velocity structure of the African continent and its relationship to the surface geology. The distribution of seismographs and earthquakes providing seismograms for this study results in good fundamental and higher mode path coverage by a large number of relatively short propagation paths, allowing us to image the SV-wave speed structure, with a horizontal resolution of several hundred kilometres and a vertical resolution of similar to 50 km, to a depth of about 400 km. The difference in mantle structure between the Archean and Pan-African terranes is apparent in our African upper-mantle shear wave model. High-velocity (4-7 per cent) roots exist beneath the cratons. Below the West African, Congo and Tanzanian Cratons, these extend to 225-250 km depth, but beneath the Kalahari Craton, the high wave speed root extends to only similar to 170 km. With the exception of the Damara Belt that separates the Congo and Kalahari Cratons, any high-speed upper-mantle lid below the Pan-African terranes is too thin to be resolved by our long-period surface wave technique. The Damara Belt is underlain by higher wave speeds, similar to those observed beneath the Kalahari Craton. Extremely low SV-wave speeds occur to the bottom of our model beneath the Afar region. The temperature of the African upper mantle is determined from the SV-wave speed model. Large temperature variations occur at 125 km depth with low temperatures beneath west Africa and all of southern Africa and warm mantle beneath the Pan-African terrane of northern Africa. At 175 km depth, cool upper mantle occurs below the West African, Congo, Tanzanian and Kalahari Cratons and anomalously warm mantle occurs below a zone in northcentral Africa and beneath the region surrounding the Red Sea. All of the African volcanic centres are located above regions of warm upper mantle. The temperature profiles were fit to a geotherm to determine the thickness of the African lithosphere. Thick lithosphere exists beneath all of the cratonic areas; independent evidence for this thick lithosphere comes from the locations of diamondiferous kimberlites. Almost all diamond locations occur where the lithosphere is 175-200 km thick, but they are largely absent from the regions of the thickest lithosphere. The lithosphere is thin beneath the Pan-African terranes of northern Africa but appears to be thicker beneath the Pan-African Damara Belt in southern Africa.

S wave velocity structure of the Arabian Shield upper mantle from Rayleigh wave tomography

The shear wave velocity structure of the shallow upper mantle beneath the Arabian Shield was modeled by inverting Rayleigh wave phase velocity measurements between 45 and 140 s together with previously published Rayleigh wave group velocity measurements between 10 and 45 s. For measuring phase velocities, we applied a modified array method to data from several regional networks that minimizes the distortion of raypaths caused by lateral heterogeneity. The new shear wave velocity model shows a broad low‐velocity region to depths of ∼150 km in the mantle across the Shield and a narrower low‐velocity region at depths ≥150 km localized along the Red Sea coast and Makkah‐Madinah‐Nafud (MMN) volcanic line. The velocity reduction in the upper mantle corresponds to a temperature anomaly of ∼250–330 K. These findings, in particular the region of continuous low velocities along the Red Sea and MMN volcanic line, do not support interpretations for the origin of the Cenozoic plateau uplift and volcanism on the Shield invoking two separate plumes. When combined with images of the 410 and 660 km discontinuities, body wave tomographic models, a S wave polarization analysis, and SKS splitting results for the Arabian Peninsula, the anomalous upper mantle structure in our new velocity model can be attributed to an upwelling of warm mantle rock originating in the lower mantle under Africa that crosses through the mantle transition zone beneath Ethiopia and moves to the north and northwest under the eastern margin of the Red Sea and the Arabian Shield. In this interpretation, the difference in mean elevation between the Arabian Platform and Shield can be attributed to isostatic uplift caused by heating of the lithospheric mantle under the Shield, with the significantly higher elevations along the Red Sea coast possibly resulting also from lithospheric thinning and dynamic uplift.

Upper mantle discontinuity structure beneath East Anatolian Plateau (Turkey) from receiver functions

Imaging of seismic velocity discontinuities with receiver functions suggests a complex mantle structure beneath the East Anatolian Plateau. By stacking all the receiver functions as a function of ray parameter, we examined the moveout characteristics of the data set and tested different velocity models to fit the arrival times of converted phases from the Moho and the lithosphere–asthenosphere boundary and their multiples. The best fitting model shows anomalously thin crust (40 km) and lithosphere (65 km), and low uppermost mantle velocities causing systematic delays on later discontinuity arrivals. These findings are consistent with the high plateau supported by partially molten and buoyant asthenosphere. The upper mantle discontinuities are all visible in different frequencies (0.15 and 0.4 Hz). Especially, the 410 km discontinuity does not show frequency dependence in amplitude and suggests a sharpness of 10 km or less. Spatial variations of the 410 and 660 km discontinuities reveal anticorrelated topography and distinct zones of diminished amplitude associated with a detached slab and delaminated fragments of lithospheric mantle. In the south, the slab becomes deeper and horizontally deflected towards the east suggesting westward migration of slab detachment and resistance to slab penetration at the 660 km discontinuity. In the north, the mantle transition zone is characterized by two spatially limited lithospheric fragments with complex geometry. At the center of the study area, the transition zone is thin (230 km) suggesting excess temperature of 120 °C. This warm mantle anomaly most likely is due to hot material trapped within the upper mantle between slab in the south and delaminated fragments in the north.

A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume origin

Common geological, geochemical, and geophysical characteristics of continental fragments of East Gondwana and adjacent oceanic lithosphere define a long‐lived, low‐volume, diffuse alkaline magmatic province (DAMP) encompassing the easternmost part of the Indo‐Australian Plate, West Antarctica, and the southwest portion of the Pacific Plate. A key to generating the Cenozoic magmatism is the combination of metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, in contrast to large igneous provinces where mantle temperatures are presumed to be high. The SW Pacific DAMP magmatism has been conjecturally linked to rifting, strike‐slip faulting, mantle plumes, or hundreds of hot spots, but all of these associations have flaws. We suggest instead that sudden detachment and sinking of subducted slabs in the late Cretaceous induced Rayleigh‐Taylor instabilities along the former Gondwana margin that in turn triggered lateral and vertical flow of warm Pacific mantle. The interaction of the warm mantle with metasomatized subcontinental lithosphere that characterizes much of the SW Pacific DAMP concentrates magmatism along zones of weakness. The model may also provide a mechanism for warming south Pacific mantle and resulting Cenozoic alkaline magmatism, where the oceanic areas are characterized primarily, but not exclusively, by short‐lived hot spot tracks not readily explained by conventional mantle plume theory. This proposed south Pacific DAMP is much larger and longer‐lived than previously considered.

Crust and upper mantle shear wave structure of the southwest United States: Implications for rifting and support for high elevation

Surface wave phase velocities from 29 earthquakes are used to map the shear velocity structure to ∼350 km depth across the 950‐km‐long Rio Grande Rift Seismic Transect Experiment (LA RISTRA) seismic array in the southwest United States. Events from a range of back azimuths minimize the effects of multipathing. The resulting velocity model reveals a transition in lithospheric thickness from 200 km in the Great Plains to 45–55 km beneath the Rio Grande Rift, thickening beneath the Colorado Plateau to 120–150 km. The upper mantle low‐velocity signature of the rift is roughly twice the width of its surface morphology. An asthenospheric low‐velocity channel underlies the region west of the Great Plains and extends to 300 km depth. This channel is likely the result of warm mantle infill behind the sinking Farallon plate. Buoyant forces within this channel are sufficient to support much of the high elevation of the rift and plateau. No evidence for a deep mantle source is found beneath the rift, implying that present rifting is not driven by deep mantle upwelling. Velocities from 55 to 90 km beneath the rift axis are 10% slower than beneath the Great Plains, consistent with small amounts of partial melt. Low velocities extend to 200–300 km depth on either side of the rift but not directly beneath it, forming an inverted‐U shape. This feature may reflect mantle that has cooled through vertical advection in a subadiabatic environment. Upwelling may be reinforced by small‐scale convection caused by variations in lithospheric thickness and shallow mantle temperatures.

Seismic imaging of the downwelling Indian lithosphere beneath central Tibet.

A tomographic image of the upper mantle beneath central Tibet from INDEPTH data has revealed a subvertical high-velocity zone from approximately 100- to approximately 400-kilometers depth, located approximately south of the Bangong-Nujiang Suture. We interpret this zone to be downwelling Indian mantle lithosphere. This additional lithosphere would account for the total amount of shortening in the Himalayas and Tibet. A consequence of this downwelling would be a deficit of asthenosphere, which should be balanced by an upwelling counterflow, and thus could explain the presence of warm mantle beneath north-central Tibet.

Effect of near‐ridge thermal anomalies on the subsidence of the oceanic lithosphere: Constraints from a 2‐D dynamic model

A 2‐D dynamic cooling model with temperature‐dependent asthenosphere viscosity is devised to examine the impact of near‐ridge thermal anomalies on the subsidence of the oceanic lithosphere. In this model, mid‐ocean ridge segments with cooler than ambient mantle temperatures have calculated subsidence rates that cannot be differentiated from normal ridge segments. This occurs because the cooler thermal anomaly cannot be sustained, nor can it spread off‐axis, due to thermally driven near‐ridge buoyant flow. When the cooler anomaly is deeply rooted, this buoyant flow can be inhibited only when the asthenosphere viscosity is as high as 3×1020 Pa‐s. On the other hand, ridge segments with warmer mantle temperatures have significantly higher subsidence rates than those predicted from standard conductive cooling models. Simply varying the injection temperature invalidates the 1‐D simplification usually invoked implicitly in models of the cooling process of the oceanic lithosphere.

The role of hydraulic fractures and intermediate-depth earthquakes in generating subduction-zone magmatism

The presence of magmatism and intermediate-depth (70–300 km deep) seismicity at subduction zones is at first sight surprising. Magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle. The current model for subduction-zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge, leading to a reduction in melting temperature and magmatism1, 2, 3, 4. But there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge. The occurrence of intermediate-depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths. One proposal put forward to explain intermediate-depth seismicity is that high pore-pressure might facilitate faulting by decreasing the friction5, 6, 7. The hypothesis presented here is that non-percolating water provides the high pore-pressure, that the consequent faulting temporarily interconnects the water pores and, when a sufficient vertical height of water is interconnected, a hydrofracture is produced which transports the water out into the mantle wedge, thereby generating subduction-zone magmatism.

A Subduction Primer for Instructors of Introductory-Geology Courses and Authors of Introductory-Geology Textbooks

We need to find better ways to explain subduction to students in introductory-geology classes. Students' grasp of plate tectonics and earth-system science depends on a basic understanding of this fundamental and unique earth process. Most subduction-zone models currently presented in introductory-geology texts are obsolete and inconsistent with modern understanding of the subduction process. A better model, which emphasizes the importance of lithospheric density and melting of the overlying mantle, is outlined here. Old lithosphere is relatively cold and thick and is denser than underlying asthenospheric mantle, whereas young lithosphere is warm and thin and is less dense than asthenosphere. Because of the density differences, old lithosphere subducts easily whereas young lithosphere does not, and subduction zones involving young lithosphere behave differently than those subducting old lithosphere. Conductive heating of the lithosphere in the subduction zone is slow compared to subduction rates, with the result that subduction zones define broad regions of unusually low temperature in the mantle. Subducting lithosphere drags the overlying mantle as it sinks, inducing convection in the sub-arc mantle. Melting of the subducted plate occurs only if very young crust is subducted. In the normal case, arc volcanism happens because water is sweated out of the subducted plate and rises into the overlying, convecting, warm mantle, where the addition of water greatly lowers the temperature of mantle melting.

Magnetoconvection and thermal coupling of the Earth's core and mantle

Numerical calculations of finite amplitude magnetoconvection in a rotating spherical shell are used to investigate the thermal coupling of the Earth’s core to the mantle. From the observed distribution of lower mantle seismic velocity heterogeneity we construct a pattern of heat flow on the core-mantle boundary consisting of a spherical average q 0 plus heterogeneity with amplitude Δ q For Δ q / q 0 = 0, corresponding to a homogeneous lower mantle, convection in the presence of a strong toroidal magnetic field consists of nearly axisymmetric magnetostrophic flow inside the inner-core tangent cylinder and a single large-scale spiraling columnar plume outside the tangent cylinder. Interaction of the columnar plume with the toroidal field induces patches of radial magnetic field distributed symmetrically with respect to the equator. For Δ q / q 0 = 10, corresponding to a strongly heterogeneous lower mantle, stably stratified regions develop below warm mantle and enhanced convection develops below cold mantle. This modulation of the convection pattern breaks the columnar structure of core motions and destroys the equatorial symmetry of the induced magnetic field, without locking it to the mantle. Our results indicate that mantle structure is of secondary importance, compared with rotation, in controlling the structure of the geomagnetic field.

Early Cretaceous Sr‐rich silicic magmatism by slab melting in the Kitakami Mountains, northeast Japan

Early Cretaceous plutonic rocks in the Kitakami Mountains, northeast Japan, related to the westward subduction along the eastern margin of the East Asian continent, intruded the Jurassic accretionary complex and a microcontinental block. These plutonic rocks are classified into types A and B. The former is further subdivided into high‐Sr, low‐Sr, and intermediate series. High‐Sr series rocks occur as the central parts of three zoned plutons in the North Kitakami belt; named the Hashigami, Tanohata, and Miyako plutons. The high‐Sr series rocks show distinct chemical characteristics compared with those of the low‐Sr series. In particular, the high‐Sr series is characterized by SiO2 greater than 61 wt %, higher Na2O and lower K2O concentrations, and slightly higher Al2O3 and lower total FeO and MgO concentrations. In trace element chemistry, the high‐Sr series rocks are richer in Sr and Ga, and poorer in Rb, Pb, and Y. These characteristics resemble those of the high‐Al2O3 trondhjemite, tonalite, and dacite (high‐Al TTD). Mass balance calculation indicates that the high‐Sr series magma can be derived by partial melting of hypothetical oceanic crust (mid‐oceanic ridge basalt (MORB), plus 5% altered MORB, plus 5% sediments) leaving garnet, clinopyroxene, quartz, rutile, and apatite as restite. On the basis of the phase relations for tholeiitic basalt and the estimated geothermal gradient of the subducted slab, slab melting is considered to have occurred at a depth of 2.0 to 2.2 GPa (about 70 to 80 km), and the age of subducted oceanic crust may be younger than 10 to 25 Ma. The coexistence of the magma derived from the subducted slab with that derived from the mantle wedge is a particularly important characteristic of the Early Cretaceous magmatism in the Kitakami Mountains. This suggests that the mantle beneath the Cretaceous forearc was warmer than for normal forearcs. Subduction of the Farallon‐Izanagi ridge may have caused the warm mantle.

Stabilization of external kink and axisymmetric modes in a highly elongated tokamak

Various schemes for stabilizing the external kink and axisymmetric modes are examined for a highly elongated tokamak equilibrium by using a new initial value code. The axisymmetric mode is more strongly affected by placing the conducting wall at the inboard-outboard position rather than at the top-bottom position. Using only a one-sided wall is substantially inferior to using two walls. A warm mantle in the vacuum region weakens the unstable mode in a rather insignificant way. These results are similar for the external kink mode. Optimum coverage of the plasma boundary with conducting material was also considered for both external kink and axisymmetric modes. Only about 20% coverage of the total circumference area is sufficient to significantly reduce the growth rates of both modes.

An extensional model of graben subsidence—the first stage of basin evolution

The formation of grabens in continental crust is generally associated with tensile faulting, crustal thinning, anomalously warm mantle below the rift zone and an updoming of the outer flanks of the rift zone. These features are also common to the initial stages of sedimentary basin formation and continental rupture (creating new sea floor). This similarity suggests that similar tectonic processes, but of varying duration, may be responsible for all three phenomena. In this study subsidence within continental grabens which have been formed by a sudden local stretching event is examined. Within the zone of extension two competing effects act to determine the sense (uplift or subsidence) and magnitude of initial epeirogenic movements: these are the thinning of the low-density crust and the upwelling of hot mantle material. On continental crust, depending on which effect is dominant, either rapid uplift or rapid subsidence may occur initially. Subsequently, thermally controlled subsidence occurs as the anomalously warm mantle below the rift cools. Due to lateral heat flow, subsidence occurs more rapidly in grabens than in basins and may be accompanied by a transient uplift of the graben flanks.

Heat flow and energetics of the San Andreas Fault Zone

Approximately 100 heat flow measurements in the San Andreas fault zone indicate (1) there is no evidence for local factional heating of the main fault trace at any latitude over a 1000‐km length from Cape Mendocino to San Bernardino, (2) average heat flow is high (∼2 HFU, ∼80 mW m−2) throughout the 550‐km segment of the Coast Ranges that encloses the San Andreas fault zone in central California; this broad anomaly falls off rapidly toward the Great Valley to the east, and over a 200‐km distance toward the Mendocino Triple Junction to the northwest. As others have pointed out, a local conductive heat flow anomaly would be detectable unless the frictional resistance allocated to heat production on the main trace were ≲100 bars. Frictional work allocated to surface energy of new fractures is probably unimportant, and hydrologic convection is not likely to invalidate the conduction assumption, since the heat discharge by thermal springs near the fault is negligible. Explanations for the low dynamic friction fall into two intergradational classes: those in which the fault is weak all of the time and those in which it is weak only during earthquakes (possibly just large ones). The first class includes faults containing anomalously weak gouge materials and faults containing materials with normal frictional properties under near‐lithostatic steady state fluid pressures. In the second class, weakening is caused by the event (for example, a thermally induced increase in fluid pressure, dehydration of clay minerals, or acoustic fluidization). In this class, unlike the first, the average strength and ambient tectonic shear stress may be large, ∼1 kbar, but the stress allocated to elastic radiation (the apparent stress) must be of similar magnitude, an apparent contradiction with seismic estimates. Unless seismic radiation is underestimated for large earthquakes, it is difficult to justify average tectonic stresses on the main trace of the San Andreas fault in excess of ∼200 bars. The development of the broad Coast Range heat flow anomaly southward from Cape Mendocino suggests that heat flow increases by a factor of 2 within 4 m.y. after the passage of the Mendocino Triple Junction. This passage leaves the San Andreas transform fault zone in its wake; the depth of the anomalous sources cannot be much greater than the depth of the seismogenic layer. Some of the anomalous heat may be supplied by conduction from the warmer mantle that must occur south of the Mendocino transform (where there is no subducting slab), and some might be supplied by shear heating in the fault zone. With no contribution from shear heating, extreme mantle upwelling would be required, and asthenosphere conditions should exist today at depths of only ∼20 km in the northernmost Coast Ranges. If there is an appreciable contribution from shear heating, the heat flow constraint implies that the seismogenic layer is partially decoupled at its base and that the basal traction is in the sense that resists right lateral motion on the fault(s). As a result of these basal tractions, the average shearing stress in the seismogenic layer would increase with distance from the main fault, and the seismogenic layer would offer substantial resistance to plate motion even though resistance on the main fault might be negligible. These speculative models have testable consequences.