Deep Continental Lithosphere Research Articles

An experimental study of Fe–Ni exchange between sulfide melt and olivine at upper mantle conditions: implications for mantle sulfide compositions and phase equilibria

The behavior of nickel in the Earth’s mantle is controlled by sulfide melt–olivine reaction. Prior to this study, experiments were carried out at low pressures with narrow range of Ni/Fe in sulfide melt. As the mantle becomes more reduced with depth, experiments at comparable conditions provide an assessment of the effect of pressure at low-oxygen fugacity conditions. In this study, we constrain the Fe–Ni composition of molten sulfide in the Earth’s upper mantle via sulfide melt–olivine reaction experiments at 2 GPa, 1200 and 1400 °C, with sulfide melt \(X_{{{\text{Ni}}}}^{{{\text{Sulfide}}}}=\frac{{{\text{Ni}}}}{{{\text{Ni}}+{\text{Fe}}}}\) (atomic ratio) ranging from 0 to 0.94. To verify the approach to equilibrium and to explore the effect of \({f_{{{\text{O}}_{\text{2}}}}}\) on Fe–Ni exchange between phases, four different suites of experiments were conducted, varying in their experimental geometry and initial composition. Effects of Ni secondary fluorescence on olivine analyses were corrected using the PENELOPE algorithm (Baro et al., Nucl Instrum Methods Phys Res B 100:31–46, 1995), “zero time” experiments, and measurements before and after dissolution of surrounding sulfides. Oxygen fugacities in the experiments, estimated from the measured O contents of sulfide melts and from the compositions of coexisting olivines, were 3.0 ± 1.0 log units more reduced than the fayalite–magnetite-quartz (FMQ) buffer (suite 1, 2 and 3), and FMQ − 1 or more oxidized (suite 4). For the reduced (suites 1–3) experiments, Fe–Ni distribution coefficients \(K_{{\text{D}}}^{{}}=\frac{{(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}/X_{{{\text{Fe}}}}^{{{\text{sulfide}}}})}}{{(X_{{{\text{Ni}}}}^{{{\text{olivine}}}}/X_{{{\text{Fe}}}}^{{{\text{olivine}}}})}}\) are small, averaging 10.0 ± 5.7, with little variation as a function of total Ni content. More oxidized experiments (suite 4) give larger values of KD (21.1–25.2). Compared to previous determinations at 100 kPa, values of KD from this study are chiefly lower, in large part owing to the more reduced conditions of the experiments. The observed difference does not seem attributable to differences in temperature and pressure between experimental studies. It may be related in part to the effects of metal/sulfur ratio in sulfide melt. Application of these results to the composition of molten sulfide in peridotite indicates that compositions are intermediate in composition (\(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}\) ~ 0.4–0.6) in the shallow mantle at 50 km, becomes more Ni rich with depth as the O content of the melt diminishes, reaching a maximum (0.6–0.7) at depths near 80–120 km, and then becomes more Fe rich in the deeper mantle where conditions are more reduced, approaching (\(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}\) ~ 0.28) > 140 km depth. Because Ni-rich sulfide in the shallow upper mantle melts at lower temperature than more Fe-rich compositions, mantle sulfide is likely molten in much of the deep continental lithosphere, including regions of diamond formation.

A “high 4He/3He” mantle material detected under the East Pacific Rise (15°4′N)

AbstractWe investigate in details helium isotope data reported in Mougel et al. (2014) for 14 basaltic samples collected on the East Pacific Rise by submersible (15°4′N) where the ridge interacts with the Mathematician seamounts. Samples locations are separated by only few hundred meters across a 15 km along‐axis profile. The data reveal a strong geochemical variability that has never been observed at such high spatial resolution for helium isotope compositions. Moreover, they reveal an unusually high 4He/3He mantle component also characterized by unradiogenic lead, atypical in oceanic basalts. He‐Pb systematics suggests a mixture between a nonradiogenic lead and radiogenic helium pyroxenitic component, recycled from the deep continental lithosphere and the ambient peridotitic mantle. The He isotope difference between these two end‐members can be interpreted as a time evolution of two distinct mantle sources after a slight (U + Th)/3He fractionation, likely due to some ancient degassing during the formation of deep continental pyroxenites.

Earth's evolving stress state and the past, present, and future stability of cratonic lithosphere

The vigour of convection in the Earth's mantle declines over time because of the decay of internal heat sources. Decaying heat sources alone would imply a gradual thermal change from the Archaean to the present. The larger impact for the Earth is due to the temperature dependence of mantle viscosity. As the mantle cools, viscosity exponentially increases. This is the dominant effect that leads to a decrease in the ratio of the forces that drive convection relative to those that resist it (i.e. to a decreasing mantle Rayleigh number). Provided that the mantle remains in the high Rayleigh number regime, as it is at present, increasing viscosity outweighs declining convective velocities in determining convection-generated stress, and mantle stress levels increase from the Archaean to the present. This is demonstrated by thermal history calculations and numerical simulations of mantle convection in a plate-tectonic regime. Thermal modelling studies further predict that bulk mantle viscosity will adjust to the Earth's cooling faster than deep continental lithosphere. This results in a greater coupling between the mantle and continental lithosphere over time. Collectively these arguments lead to the conclusion that sections of continental lithosphere that have remained stable since the Archaean and the Proterozoic are becoming progressively more prone to instability in the geologically modern era.

Long memory of mantle lithosphere fabric — European LAB constrained from seismic anisotropy

We present a uniform updated model of the European lithosphere–asthenosphere boundary (LAB) recalculated from data collected during our regional studies of seismic anisotropy and other tomographic experiments, and show results of mapping of large-scale domains of mantle lithosphere characterized by uniform fossil fabrics. Exploiting the long memory of the fabric of the deep continental lithosphere, we define the LAB as a boundary between a fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy related to present-day flow in the asthenosphere. Analysis of static terms of teleseismic P-wave travel time deviations shows that the LAB topography is more distinct beneath the Phanerozoic part of Europe than beneath its Precambrian part and deepens down to ∼ 220 km beneath the two Alpine roots, the South Carpathians and eastward of the Trans-European Suture Zone. Separation of the relative P residuals into static and directional-dependent terms, shear-wave splitting analysis and joint inversion of anisotropic parameters, reveals that the mantle lithosphere consists of domains with consistent olivine fabrics, which can be modelled by peridotite aggregates with plunging foliation or lineation. Changes of the fossil fabric allow us to map the domain boundaries within the mantle lithosphere. We propose processes which could create the observed fossil fabric as a consequence of successive subduction and accretion of micro-continent fragments outboard of continental cratons and a gradual stabilization of the LAB by a mantle flow after a detachment of the lower parts of subducting slabs. We advocate the importance of combining different methods and data in Earth structure studies and applying 3D approaches considering both seismic anisotropy and the general orientation of symmetry axes to construct more realistic models of large-scale lithospheric structure.

Stagnant lid convection and carbonate metasomatism of the deep continental lithosphere

The current (and past) mantle adiabat beneath continents is hot enough that ascending carbonate‐rich domains partially melt liberating kimberlites. Over the last 2 Ga, most of these kimberlites have been trapped and frozen in the deep continental lithosphere, forming a massive CO2 reservoir. Geochemical studies of kimberlites that reached the surface indicate the pervasiveness of their effects within the deep lithosphere. These magmas extensively reacted with the deep lithosphere on the way up. Conversely, studies of deep mantle xenoliths brought up in kimberlites detect repeated episodes of metasomatism by these fluids. The heat balance between convection and conduction through the lithosphere provides gross constraints on the cumulative global effects of this process. Stagnant lid (including chemical lid) convection supplies heat to the base of continental lithosphere in equilibrium with the conductive heat flow. The thermal gradient beneath continents is constrained by xenolith geotherms and mantle heat flow q is ∼20 mW m−2. The upwelling velocity V at the base of the thermal boundary layer is ∼q/TηρC, where ρC is volume specific heat, 4 MJ m−3 and Tη is the temperature to change viscosity by a factor of e, ∼60 K. The upwelling velocity is thus ∼2 km per million years; the mass of the mantle circulates beneath continents over ∼2 Ga. The rate that plate tectonics recirculates the mantle is comparable and limits the supply carbonate‐rich undepleted material to the melting zone of upwelling stagnant lid convection and thus the production of kimberlites. Overall the amount of CO2 emplaced by kimberlites into continental lithosphere is a significant fraction (∼1/2) of that in the convecting mantle, resulting in a major deep lithospheric reservoir of CO2 of ∼5000 × 1018 moles, an equivalent thickness of ∼2 km carbonatized rock. This process, however, is insufficient to affect the overall density of deep lithosphere. The observed linearity of the xenolith geotherm precludes equivalent thicknesses of several kilometers depending on the radioactive element concentration of actual kimberlites. Kimberlites are partial melts often of subducted carbonatized oceanic crust and hence have CO2 to radioactive element ratios higher than the bulk mantle. Most kimberlites remain within the deep lithosphere because ascending dikes do not usually penetrate the region of horizontal compression in the strong cool part of the lithosphere. Conversely stresses relax quickly within the warm deep lithosphere allowing dikes to repeatedly intrude.

Water in minerals of the continental lithospheric mantle and overlying lower crust: A comparative study of peridotite and granulite xenoliths from the North China Craton

Water contents of clinopyroxene, orthopyroxene and olivine in mantle peridotite xenoliths from the Hannuoba and Nushan basaltic volcanoes (North China Craton) were determined by Fourier transform infrared spectroscopy (FTIR). Results show that contents vary from 50 to 155 ppm (H 2O wt.) for clinopyroxene and from 20 to 55 ppm for orthopyroxene in Hannuoba peridotites, and from 5 to 355 ppm for clinopyroxene and from 5 to 140 ppm for orthopyroxene in Nushan peridotites. Water contents in clinopyroxene and orthopyroxene define a good positive correlation with a ratio of ~ 2.1. This, together with the roughly homogeneous distribution of hydrogen from core to rim region in pyroxene grains, suggests that they have preserved the initial water content of the mantle source. By contrast, olivine in these peridotites contain very little, if any, water, which were probably caused by significant loss of H during their ascent due to the faster H diffusion in olivine relative to pyroxenes and by the relatively lower water content in the mantle source of Hannuoba and Nushan. Previous studies [Xia, Q.K., Yang, X.Z., Deloule, E., Sheng, Y.M., Hao, Y.T., 2006. Water in the lower crustal granulite xenoliths from Nushan, eastern China. Journal of Geophysical Research, 111(B11202) doi:10.1029/2006JB004296; Yang, X.Z., Deloule, E., Xia, Q.K., Fan, Q.C., Feng, M., in press. Water contrast between Precambrian and Phanerozoic continental lower crust of eastern China. Journal of Geophysical Research doi:10.1029/2007JB005541] have shown that the water content ranges from 275 to 720 ppm for clinopyroxene, from 60 to 185 ppm for orthopyroxene and from 65 to 205 ppm for plagioclase in Hannuoba mafic granulite xenoliths, and from 200 to 2330 ppm for clinopyroxene, from 140 to 1270 ppm for orthopyroxene and from 145 to 900 ppm in plagioclase in Nushan mafic granulite xenoliths. The significant difference between the water content of peridotite and that of coexisting mafic granulite xenoliths, e.g. 0–100 ppm (mostly < 50 ppm) vs. 100–1000 ppm (mostly > 200 ppm) for the estimated bulk concentration, demonstrates that the distribution of water in the deep continental lithosphere is vertically heterogeneous. This may be due to the different genesis of peridotites and granulites (the residue of partial melting vs. the product of melt crystallization). Based on these water data, we define the rheological structure of the lowermost crust and uppermost mantle below the Hannuoba and Nushan regions. The viscosity (strength) of the underlying shallow mantle is similar, whereas the deep crust is relatively stronger at Hannuoba and weaker at Nushan. Difference in the viscosity structure may further influence the dynamical processes between different zones in this craton.

Seismic anisotropy - a method for studying the fabric of deep continental lithosphere

Independent seismological results, obtained from shear-wave splitting, azimuthal variation of Pn velocities, P-residual spheres and dispersion of surface waves, have demonstrated to observed anisotropies originates in preferred orientation of olivine crystals in the mantle lithosphere. In comparison with oceanic regions, the large-scale fabric of the continental lithosphere is more complex. While the surface-wave data provide an integral property of a number of tectonic units, results on splitting of sub-vertically incident teleseismic shear waves show consistent orientations of the fast S waves within individual lithospheric blocks. In many provinces, the mean azimuth of the fast S polarization parallels the regional geological fabric at the surface (e.g. the strike of a mountain belt). The azimuth-incidence angle dependent terms of teleseismic P-wave residuals, cleared of the effects of source regions and deep mantle paths, show a systematic directional dependence of the relative residuals over distances of several hundred kilometres. Abrupt changes in orientations of the high- and low-velocity directions are related to important tectonic boundaries (deep-seated suture zones). The continental lithosphere related anisotropy scanned by P waves requires a tectonic block structure with dipping high- and low-velocity directions within the lithosphere. Present-day models and interpretations of individual observations of seismic anisotropy depend on the wavelength of different types of seismic waves, as well as on directions of the propagation (both as to azimuths and dips from the horizontal). A joint analysis of body waves - both anisotropic P-residual spheres and shear-wave splitting - allows detecting large-scale anisotropic structures approximated either by hexagonal or orthorhombic symmetries with inclined symmetry axes.

Lower crustal and upper mantle xenoliths along the Cheyenne belt and vicinity

Lower-crustal and upper-mantle xenoliths entrained in Phanerozoic igneous rocks in the northern Southern Rocky Mountains provide the only direct means of studying the petrologic and geochemical nature of the deep-continental lithosphere in this region. Petrographic and geochemical features of peridotite xenoliths from the State Line kimberlite district, near the Cheyenne belt, suggest that during Devonian kimberlite emplacement, infertile, cryptically metasomatized, Archean mantle dipped south beneath a distinctly more fertile, and veined, lithospheric mantle associated with the Proterozoic continental crust of the Colorado province. Lower-crustal xenoliths recovered from the State Line kimberlites and late Cenozoic ultrapotassic igneous rocks at Leucite Hills (north of the Cheyenne belt) are dominated by mafic granulites. This indicates that both the Archean and Proterozoic lower-continental crust was, and may remain today, mafic in composition. Existing major-element data support a tholeiitic basalt precursor for the mafic granulites associated with the Colorado province. Unfortunately, despite its importance for constraining models of lithospheric structure in the northern Rocky Mountains, detailed information regarding the age, trace element and isotopic compositions, or physical attributes of the mafic xenoliths is presently not available. In order to evaluate the age and composition of continental lithosphere in this region and to assess the possible role of mafic lower-crustal rock types as sources for Laramide magmatism we outline a plan (as part of the Rocky Mountain Continental Dynamics project) to obtain geochemical and geochronological information from xenoliths recovered at the State Line kimberlite district and Leucite Hills.

Experimental tests of garnet peridotite oxygen barometry

We have performed experiments aimed at testing the calibration of oxygen barometers for the garnet peridotite [garnet (Gt)-olivine (Ol)-orthopyroxene (Opx)] phase assemblage. These involved equilibrating a thin layer of garnet sandwiched between layers of olivine and orthopyroxene at 1300°C and 23–35 kbar for 1–7 days. Oxygen fugacity was controlled (but not buffered) by using inner capsules of Fe−Pt alloy or graphitc or molybdenum sealed in welded Pt outer capsules. Post-experiment measurement of fO2 was made by determining the compositions of Pt-Fe alloy sensors at the interface between garnet and olivine + orthopyroxene layers. The composition of alloy in equilibrium with olivine + orthopyroxene was approached from Fe-oversaturated and Fe-undersaturated conditions in the same experiment with, in general, excellent convergence. Product phase compositions were determined by electron microprobe and a piece of the garnet layer saved for 57Fe Mossbauer spectroscopy. The latter gave the Fe3+ content of the garnet at the measured P-T-fO2 conditions. Approach to equilibrium was checked by observed shifts in Fe3+ content and by the approach of garnet-olivine Fe−Mg partitioning to the expected value. The compositions of the phases were combined with mixing properties and thermodynamic data to calculate an apparent fO2 from two possible garnet oxybarometers:- (1) $$\begin{gathered} 2Ca_3 Fe_2 Si_3 O_{12} + 2Mg_3 Al_2 Si_3 O_{12} + 4FeSiO_3 = 2Ca_3 Al_2 Si_3 O_{12} \hfill \\ Gt Gt Opx Gt \hfill \\ + 8FeSi_{0.5} O_2 + 6MgSiO_3 + O \hfill \\ Ol Opx \hfill \\ \end{gathered} $$ and (2) $$\begin{gathered} 2Fe_3 Fe_2 Si_3 O_{12} = 8FeSi_{0.5} O_2 + 2FeSi_3 O_2 \hfill \\ Gt Ol Opx \hfill \\ \end{gathered} $$ Comparison of calculated fO2s with those measured by the Pt-Fe sensors demonstrated that either barometer gives the correct answer within the expected uncertainty. Data from the first (Luth et al. 1990) has an uncertainty of about 1.6 logfO2 units, however, while that from equilibrium (2) (Woodland and O'Neill 1993) has an error of +/- 0.6 log units, comparable to that of the spinel peridotite oxybarometer. We therefore conclude that equilibrium (2) may be used to calculate the fO2 recorded by garnet peridotites with an uncertainty of about +/- 0.6 log units, providing the potential to probe the oxidation environment of the deep continental lithosphere. Preliminary application based on data from Luth et al. (1990) indicates that garnet peridotite xenoliths from Southern Africa record oxygen fugacities about 3.0 log units below the FMQ (fayalite-magnetite-quartz) buffer. These are substantially more reducing conditions than those recorded by continental spinel lherzolites which typically give oxygen fugacities close to FMQ (Wood et al. 1990).

Late Cenozoic uplift and volcanism on Spitsbergen: Caused by mantle convection?

Regional tectonic uplift, heat-flow measurements, and P-T estimates on upper-mantle xenoliths associated with Neogene volcanic rocks indicate a strongly thinned (∼50 km thick) lithosphere beneath Svalbard, 650 km north of Scandinavia. Data on Neogene volcanic rocks and xenoliths indicate an ∼50 °C temperature anomaly in the underlying asthenosphere, together with volatile-rich conditions in the upper mantle. The Svalbard uplift is one of several Cenozoic epeirogenic uplifts along the margins of the North Atlantic. Boundary processes occurring where hot-spot-influenced asthenosphere abuts cold, deep continental lithosphere seem to be likely explanations for the uplifts.

Tectonic development of passive continental margins of the southern and central Red Sea with a comparison to Wilkes Land, Antarctica

The continental margins of the southern and central Red Sea and most of Wilkes Land, Antarctica have bulk crustal configurations and detailed structures that are best explained by a prolonged history of magmatic expansion that followed a brief, but intense period of mechanical extension. Extension on the Red Sea margins was spatially confined to a rift that was 20–30 km in width. The rifting phase along the Arabian margin of the central and southern Red Sea occurred 25–32 Ma ago, primarily by detachment faulting at upper crustal levels and ductile uniform stretching at depth. Rifting was followed by an early magmatic phase during which the margin was invaded by dikes and plutons, primarily of gabbro and diorite, at 20–24 Ma, after the crust was mechanically thinned from 40 km to ≈ 20 km. We infer continued spreading after that in which broad shelves were formed by a process of magmatic expansion, because the offshore crust is only 8–15 km thick, including sediment, and seismic reflection data do not depict horst and graben or half graben structures from which mechanical extension might be inferred. The Wilkes Land margin is similar to the Arabian example. The margin is about 150 km in width, the amount of upper crustal extension is too low to explain the change in sub-sediment crustal thickness from ≈ 35 km on the mainland to < 10 km beneath the margin and reflectors in the deepest seismic sequence are nearly flat lying. Our model requires large volumes of melt in the early stages of continental rifting. The voluminous melt might be partly a product of nearby hot spots, such as Afar and partly the result of an initial period of partial fusion in the deep continental lithosphere under lower temperatures than ordinarily required by dry solidus conditions.

The timing of uplift, volcanism, and rifting peripheral to the Red Sea: A case for passive rifting?

Prior to the formation of the Red Sea the northeastern Afro/Arabian continent had low relief and was largely below sea level from the Late Cretaceous to the early Oligocene. The events leading to the formation of the Red Sea followed the sequence (1) alkaline volcanism and rifting beginning about 30–32 Ma affecting a narrow linear zone in the continent, (2) rotational block faulting and detachment faulting, well underway by 25 Ma, (3) gabbro and diorite magmatism, andesite to rhyolite volcanism, and fine‐grained nonmarine sedimentation in the rift between 20 and 25 Ma, (4) fine‐grained marine sedimentation in the rift as the early shelves started to subside in the middle Miocene, and (5) uplift of the adjacent continents (about 3 km) and subsidence of the shelves (about 4 km) between 13.8 and 5 Ma. The youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the Proterozoic Arabian Shield that range in age from 13.8 to 568 Ma. The youngest of these ages, coupled with the present high relief along the Arabian escarpment and published heat flow measurements, indicate that 2.5–4 km uplift has occurred in the last 13.8 m.y. The sequence volcanism/rifting followed by uplift leads to our adoption of a passive mantle model for rift origin. Models that require uplift to create the rift are rejected, because of the late uplift. We advocate a model of lithospheric extension caused by two‐dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere. It is acknowledged that traction models could explain the observed data, but they imply a rigid, static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions. Neither requirement is necessary in the extension model. The rift starts with mechanical extension in a narrow zone of lithosphere between 25–32 Ma in our model. The thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift. Ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis. Once partially melted, rocks formerly part of the continental lithosphere join the upwelling asthenosphere, resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge. The resulting density decrease explains the uplift. We think that the Red Sea began as a consequence of changing plate geometries resulting from the collision of India and Eurasia. After the collision, the segment of the Owens fracture zone north of the Carlsberg Ridge became locked, forcing the northeast corner of Afro/Arabia to rotate with the Indian plate away from the rest of Africa.

Empirical geothermobarometry for garnet peridotites and implications for the nature of the lithosphere, kimberlites and diamonds

As a result of a study on the influence of Cr on geothermometry and geobarometry empirical garnet-orthopyroxene geobarometry has been refined. The applicability to simple and complex systems is demonstrated and the modelling procedure briefly outlined. The suitability of some geothermometers for application to garnet lherzolite xenoliths and their limits of precision are discussed. Fields of pressure-temperature estimates for garnet lherzolites from various provinces are shown. Low-temperature xenoliths (< 1050°C) give estimates in accord with a conductive geotherm for the continental lithosphere. African xenoliths (both South African and “off-craton” Namibian) record a distinctive pattern, exhibiting a “temperature discontinuity”, where high-temperature xenoliths (> 1050°C) give near-isobaric estimates for a range of temperatures. The depth of this “temperature discontinuity” is shallower for off-craton than for on-craton suites. Similar patterns are indicated for suites from U.S.A. and the U.S.S.R. The temperature discontinuity is interpreted as a geologically short-lived transient state, where the deepest parts of the lithosphere are in the process of adjustment to higher heat flux from below. At the highest temperatures recorded by those suites “wet” peridotitic solidi are intersected and thus the xenoliths record events, were P,T conditions may have allowed the formation of kimberlitic or strongly undersaturated magmas at about the depths of this discontinuity. This process is inferred to lead to thermal erosion of the base of the lithosphere. The generation and subsequent crystallization of magmas at the base of the lithosphere is inferred to be the source for diamond formation. The distinctive pattern of high- and low-temperature xenoliths in relation to the diamond stability field shows the existence of a rather small “diamond window” between about 900 and 1300°C at 40–55 kb. This window exists only underneath old, stable continents with a thick lithosphere. If recent age determinations on diamonds are correct, this implies the existence of thick continental lithosphere with similar characteristics to present-day lithosphere in the Archean. A model is presented arguing for hot, upwelling convection streams as a possible source of heat. The model implies differences in lithospheric development according to the speed of plates, carrying a continent: Continuous lithospheric thinning by thermal erosion for steady or very slow moving plates with the break-up of a continent and the formation of an ocean as one extreme, the continuous repetition of thermal erosion and growth on cooling (underplating) of the continental lithosphere as the other extreme for fast moving plates. The latter may be the cause of a number of complexities and heterogeneities both within the deep continental lithosphere as well as in the convecting mantle.