Underlying Mantle Flow Research Articles

Convective Self‐Compression of Cratons and the Stabilization of Old Lithosphere

AbstractDespite being exposed to convective stresses for much of the Earth's history, cratonic roots appear capable of resisting mantle shearing. This tectonic stability can be attributed to the neutral density and higher strength of cratons. However, the excess thickness of cratons and their higher viscosity amplify coupling to underlying mantle flow, which could be destabilizing. To investigate the stresses that a convecting mantle exerts on cratons that are both strong and thick, we developed instantaneous global spherical numerical models that incorporate present‐day geoemetry of cratons within active mantle flow. Our results show that mantle flow is diverted downward beneath thick and viscous cratonic roots, giving rise to a ring of elevated and inwardly‐convergent tractions along a craton's periphery. These tractions induce regional compressive stress regimes within cratonic interiors. Such compression could serve to stabilize older continental lithosphere against mantle shearing, thus adding an additional factor that promotes cratonic longevity.

Global Models From Sparse Data: A Robust Estimate of Earth's Residual Topography Spectrum

AbstractA significant component of Earth's surface topography is maintained by stresses induced by underlying mantle flow. This “dynamic” topography cannot be directly observed, but it can be approximated—particularly at longer wavelengths—from measurements of residual topography, which are obtained by removing isostatic effects from the observed topography. However, as these measurements are made at discrete, unevenly distributed locations on Earth's surface, inferences about global properties can be challenging. In this paper, we present and apply a new approach to transforming pointwise measurements into a continuous global representation. The approach, based upon the statistical theory of Gaussian processes, is markedly more stable than existing approaches—especially for small data sets. We are therefore able to infer the spatial pattern, wavelength, and amplitude of residual topography using only the highest quality oceanic spot measurements within the database of Hoggard et al. (2017, https://doi.org/10.1002/2016JB013457). Our results indicate that the associated spherical harmonic power spectrum peaks at l = 2, with power likely in the range 0.46–0.76 km2. This decreases by over an order of magnitude to around 0.02 km2 at l = 30. Around 85% of the total power is concentrated in degrees 1–3. Our results therefore confirm previous findings: Earth's residual topography expression is principally driven by deep mantle flow, but shallow processes are also crucial in explaining the general form of the power spectrum. Finally, our approach allows us to determine the locations where collection of new data would most impact our knowledge of the spectrum.

Seismic evidence for the existence of an entrained mantle flow coupling the northward advancing Indian plate under Tibet

The Tibetan Plateau, known as the roof of the world” and the third pole of the earth”, is a product of the collision between India and Asia during the last ~50 Ma. The regional tectonics–in particular, growth and expansion of the plateau–has been attributed primarily to deformation within the lithosphere. The role and pattern of the underlying asthenospheric flow, however, remain mostly unaddressed. In light of recent seismic tomographic images and published seismic anisotropic descriptions of the upper mantle, here we propose that an entrained mantle flow is likely to exist under Tibet, induced by the northward advancing Indian plate. The direction of mantle flow is characterized by a gradual rotation from northward in the south to eastward in the north as a result of deflection by the deep root of the Tarim block. The presence of an underlying mantle flow is not only able to account for the west-east oriented fast-axis of seismic anisotropy in northern Tibet, but can also adequately explain the sporadic null splitting in southern Tibet. Specifically, the null splitting results, at least in part, from upwellings of asthenospheric flow through tears of the underthrusting Indian plate that have been revealed by various seismic observations. The mantle flow may in turn promote the block extrusion under Tibet that has been observed in GPS measurements; hot asthenospheric upwellings may also lead to widespread post-collisional magmatism in southern Tibet.

Collapse of passive margins by lithospheric damage and plunging grain size

The collapse of passive margins has been proposed as a possible mechanism for the spontaneous initiation of subduction. In order for a new trench to form at the junction between oceanic and continental plates, the cold and stiff oceanic lithosphere must be weakened sufficiently to deform at tectonic rates. Such rates are especially hard to attain in the cold ductile portion of the lithosphere, at which the mantle lithosphere reaches peak strength. The amount of weakening required for the lithosphere to deform in this tectonic setting is dictated by the available stress. Stress in a cooling passive margin increases with time (e.g., due to ridge push), and is augmented by stresses present in the lithosphere at the onset of rifting (e.g., due to drag from underlying mantle flow). Increasing stress has the potential to weaken the ductile portion of the lithosphere by dislocation creep, or by decreasing grain size in conjunction with a grain-size sensitive rheology like diffusion creep. While the increasing stress acts to weaken the lithosphere, the decreasing temperature acts to stiffen it, and the dominance of one effect or the other determines whether the margin might weaken and collapse. Here, we present a model of the thermal and mechanical evolution of a passive margin, wherein we predict formation of a weak shear zone that spans a significant depth-range of the ductile portion of the lithosphere. Stiffening due to cooling is offset by weakening due to grain size reduction, driven by the combination of imposed stresses and grain damage. Weakening via grain damage is modest when ridge push is the only source of stress in the lithosphere, making the collapse of a passive margin unlikely in this scenario. However, adding even a small stress-contribution from mantle drag results in damage and weakening of a significantly larger portion of the lithosphere. We posit that rapid grain size reduction in the ductile portion of the lithosphere can enable, or at least significantly facilitate, the collapse of a passive margin and initiate a new subduction zone. We use this model to estimate the conditions for passive margin collapse for modern and ancient Earth, as well as for Venus.

Surface sculpting by hidden agents

Tectonic plate interiors are often regarded as relatively inactive. Yet, reconstructions of marine terrace uplift in Angola suggest that underlying mantle flow can rapidly warp Earth's surface far from obviously active plate boundaries.

Variability and origin of seismic anisotropy across eastern Canada: Evidence from shear wave splitting measurements

AbstractMeasurements of seismic anisotropy in continental regions are frequently interpreted with respect to past tectonic processes, preserved in the lithosphere as “fossil” fabrics. Models of the present‐day sublithospheric flow (often using absolute plate motion as a proxy) are also used to explain the observations. Discriminating between these different sources of seismic anisotropy is particularly challenging beneath shields, whose thick (≥200 km) lithospheric roots may record a protracted history of deformation and strongly influence underlying mantle flow. Eastern Canada, where the geological record spans ∼3 Ga of Earth history, is an ideal region to address this issue. We use shear wave splitting measurements of core phases such as SKS to define upper mantle anisotropy using the orientation of the fast‐polarization direction ϕ and delay time δt between fast and slow shear wave arrivals. Comparison with structural trends in surface geology and aeromagnetic data helps to determine the contribution of fossil lithospheric fabrics to the anisotropy. We also assess the influence of sublithospheric mantle flow via flow directions derived from global geodynamic models. Fast‐polarization orientations are generally ENE‐WSW to ESE‐WNW across the region, but significant lateral variability in splitting parameters on a ≤100 km scale implies a lithospheric contribution to the results. Correlations with structural geologic and magnetic trends are not ubiquitous, however, nor are correlations with geodynamically predicted mantle flow directions. We therefore consider that the splitting parameters likely record a combination of the present‐day mantle flow and older lithospheric fabrics. Consideration of both sources of anisotropy is critical in shield regions when interpreting splitting observations.

The dynamics of extrusion tectonics: Insights from numerical modeling

To elucidate the dynamics of extrusion tectonics, I investigate the relation between subduction processes and intraplate deformation. Three-dimensional numerical models of subducting upper plate systems show that subduction of a heterogeneous buoyancy slab, that is, hosting continental and oceanic lithosphere, causes the retreat of the oceanic trench and the localization of deformation in the back arc. Similar trench evolution is found when the subducting slab partially detaches from the impinging continent; however, deformation in the upper plate interiors widens, with indentation, plastic yielding and rotation of a large block extruding laterally toward the retreating margin. Indentation stresses are consequence of the partial slab breakoff at depth and vanish when the detached slablet reaches the mantle transition zone, resulting in transient extrusion tectonics. The distance to which indentation deformation propagates depends on the ability of subducting lithosphere to propagate stress laterally and is linear with the lithospheric strength. While in the stretching back arc the plate deformation and the underlying mantle flow are coupled, they are uncoupled in the extrusion tectonics, where mostly stresses propagated through the margin drive rotations. The potential role of these diverse subduction processes in the eastern Mediterranean is discussed, suggesting that different mechanisms likely drive the tectonics of this area.

Importance of initial buoyancy field on evolution of mantle thermal structure: Implications of surface boundary conditions

Although there has been significant progress in the seismic imaging of mantle heterogeneity, the outstanding issue that remains to be resolved is the unknown distribution of mantle temperature anomalies in the distant geological past that give rise to the present-day anomalies inferred by global tomography models. To address this question, we present 3-D convection models in compressible and self-gravitating mantle initialised by different hypothetical temperature patterns. A notable feature of our forward convection modelling is the use of self-consistent coupling of the motion of surface tectonic plates to the underlying mantle flow, without imposing prescribed surface velocities (i.e., plate-like boundary condition). As an approximation for the surface mechanical conditions before plate tectonics began to operate we employ the no-slip (rigid) boundary condition. A rigid boundary condition demonstrates that the initial thermally-dominated structure is preserved, and its geographical location is fixed during the evolution of mantle flow. Considering the impact of different assumed surface boundary conditions (rigid and plate-like) on the evolution of thermal heterogeneity in the mantle we suggest that the intrinsic buoyancy of seven superplumes is most-likely resolved in the tomographic images of present-day mantle thermal structure. Our convection simulations with a plate-like boundary condition reveal that the evolution of an initial cold anomaly beneath the Java-Indonesian trench system yields a long-term, stable pattern of thermal heterogeneity in the lowermost mantle that resembles the present-day Large Low Shear Velocity Provinces (LLSVPs), especially below the Pacific. The evolution of subduction zones may be, however, influenced by the mantle-wide flow driven by deeply-rooted and long-lived superplumes since Archean times. These convection models also detect the intrinsic buoyancy of the Perm Anomaly that has been identified as a unique slow feature distinct from the two principal LLSVPs. We find there is no need for dense chemical ‘piles’ in the lower mantle to generate a stable distribution of temperature anomalies that are correlated to the LLSVPs and the Perm Anomaly. Our tomography-based convection simulations also demonstrate that intraplate volcanism in the south-east Pacific may be interpreted in terms of shallow small-scale convection triggered by a superplume beneath the East Pacific Rise.

The role of magmatically driven lithospheric thickening on arc front migration

AbstractVolcanic activity at convergent plate margins is localized along lineaments of active volcanoes that focus rising magma generated within the mantle below. In many arcs worldwide, particularly continental arcs, the volcanic front migrates away from the interface of subduction (the trench) over millions of years, reflecting coevolving surface forcing, tectonics, crustal magma transport, and mantle flow. Here we show that extraction of melt from arc mantle and subsequent magmatic thickening of overlying crust and lithosphere can drive volcanic front migration. These processes are consistent with geochemical trends, such as increasing La/Yb, which show that increasing depths of differentiation correlate with arc front migration in continental arcs. Such thickening truncates the underlying mantle flow field, squeezing hot mantle wedge and the melting focus away from the trench while progressively decreasing the volume of melt generated. However, if magmatic thickening is balanced by tectonic extension in the upper plate, a steady crustal thickness is achieved that results in a more stationary arc front with long‐lived mantle melting. This appears to be the case for some island arcs. Thus, in combination with tectonic modulation of crustal thickness, magmatic thickening provides a self consistent model for volcanic arc front migration and the composition of arc magmas.

Passive margins getting squeezed in the mantle convection vice

[1] Passive margins often exhibit uplift, exhumation, and tectonic inversion. We speculate that the compression in the lithosphere gradually increased during the Cenozoic, as seen in the number of mountain belts found at active margins during that period. Less clear is how that compression increase affects passive margins. In order to address this issue, we design a 2-D viscous numerical model wherein a lithospheric plate rests above a weaker mantle. It is driven by a mantle conveyor belt, alternatively excited by a lateral downwelling on one side, an upwelling on the other side, or both simultaneously. The lateral edges of the plate are either free or fixed, representing the cases of free convergence, and collision (or slab anchoring), respectively. This distinction changes the upper mechanical boundary condition for mantle circulation and thus, the stress field. Between these two regimes, the flow pattern transiently evolves from a free-slip convection mode toward a no-slip boundary condition above the upper mantle. In the second case, the lithosphere is highly stressed horizontally and deforms. For a constant total driving force, compression increases drastically at passive margins if upwellings are active. Conversely, if downwellings alone are activated, compression occurs at short distances from the trench and extension prevails elsewhere. These results are supported by Earth-like models that reveal the same pattern, where active upwellings are required to excite passive margins compression. Our results substantiate the idea that compression at passive margins is in response to the underlying mantle flow that is increasingly resisted by the Cenozoic collisions.

Stability of active mantle upwelling revealed by net characteristics of plate tectonics

Viscous convection within the mantle is linked to tectonic plate motions and deforms Earth's surface across wide areas. Such close links between surface geology and deep mantle dynamics presumably operated throughout Earth's history, but are difficult to investigate for past times because the history of mantle flow is poorly known. Here we show that the time dependence of global-scale mantle flow can be deduced from the net behaviour of surface plate motions. In particular, we tracked the geographic locations of net convergence and divergence for harmonic degrees 1 and 2 by computing the dipole and quadrupole moments of plate motions from tectonic reconstructions extended back to the early Mesozoic era. For present-day plate motions, we find dipole convergence in eastern Asia and quadrupole divergence in both central Africa and the central Pacific. These orientations are nearly identical to the dipole and quadrupole orientations of underlying mantle flow, which indicates that these 'net characteristics' of plate motions reveal deeper flow patterns. The positions of quadrupole divergence have not moved significantly during the past 250 million years, which suggests long-term stability of mantle upwelling beneath Africa and the Pacific Ocean. These upwelling locations are positioned above two compositionally and seismologically distinct regions of the lowermost mantle, which may organize global mantle flow as they remain stationary over geologic time.

Mantle flow uplift of western Anatolia and the Aegean: Interpretations from geophysical analyses and geodynamic modeling

The Western Anatolian and Aegean region demonstrates a complex geologic history of horizontal and vertical tectonics. Active normal faulting and exhumation zones indicate that Western Anatolia has experienced significant extension since the Oligocene‐Early Miocene (∼30 Ma). Our geophysical analyses demonstrate that the region is also uplifted relative to an elevation that would be expected given an isostatic response to the lithosphere structure. Namely, topography “residuals” indicate a residual uplift of about 1500 m over ∼200 km sections of Western Anatolia and the Aegean. Admittance functions between free‐air gravity and topography indicate that the regional topography is isostatically uncompensated and as it approaches ∼50 mGal/km at the longest wavelengths, the uncompensated topography is likely owing to an underlying mantle flow component. Using forward geodynamic modelling we consider an idealized section of Western Anatolian lithosphere based on tomographic inversions and examine the magnitude and pattern of surface topography to reconcile with the geophysical observables. The models consistently show a plateau‐type uplift (and horizontal extension) through Western Anatolia with an amplitude and wavelength consistent with the residual topography calculations. Together, the geophysical analyses and modelling provide independent quantitative evidence that the thin Anatolian‐Aegean lithosphere is being buoyed upwards by underlying mantle flow. The mantle flow may be associated with active lithosphere delamination beneath the region; a process that would also explain the ongoing crustal extension.

Influence of past and present-day plate motions on spherical models of mantle convection: implications for mantle plumes and hotspots

SUMMARY We explore the influence of tectonic plate motions, given by the no-net-rotation (NNR)NUVEL-1 model, on a 3-D spherical model of mantle convection in which plates can be coupled to the underlying mantle flow in a dynamically consistent manner. We first derived a reference convection model in which only the NUVEL-1 geometry of the tectonic plates is prescribed. The plate rotations are then predicted on the basis of the buoyancy forces in the mantle, ensuring a dynamical balance of torques acting on the plates. This dynamically consistent reference convection model, which is based on a simple two-layer viscosity profile, yields the main features of plate tectonics: linear subduction zones, passive diverging zones and four mantle plumes. We next developed a time-dependent convection model, which is initiated with the average radial temperature field extracted from the reference convection model, and in which we imposed the NNR-NUVEL-1 plate velocities. This convection simulation yields six focused upwelling plumes, whose location and number is very similar to the primary terrestrial hotspots which have been recently identified (Courtillot et al. 2003). In all convection models incorporating the NNR-NUVEL-1 plate motions, we find that the surface heat flow and mantle potential temperature stay essentially constant, demonstrating the compatibility between the observed NNR-NUVEL-1 velocities and the internal buoyancy forces in the mantle. To determine the robustness of these results we carried out complementary convection simulations incorporating the past 120 Ma history of tectonic plate evolution. These simulations yielded shifting ‘hotlines’ at the core‐mantle boundary, but the locations of the overlying hotspot plumes remained relatively stable. The configuration of the hotlines in the convection experiments with and without evolving surface plate geometries are very similar to each other, showing that convection models with present-day plate configurations are sufficient for capturing the essential characteristics of the present-day thermal structure in the mantle. In a final experiment, the prescribed NNR-NUVEL-1 plate velocity constraint is released, allowing the plates to rotate freely in response to the underlying mantle flow. We find that the initial evolution of this model is characterized by a strong stability of the mantle thermal structures, in particular the upwelling plumes. An important and novel feature of the convection model with free plate motions is the predicted opening of the African plate along the East African Rift boundary, which occurs in response to the large-scale mantle flow and does not appear to require the presence of upwelling plumes directly beneath the rift.

Coupled crust‐mantle dynamics and intraplate tectonics: Two‐dimensional numerical and three‐dimensional analogue modeling

Tectonic deformation of some intraplate regions may be caused by the Rayleigh‐Taylor (RT) instability of dense subcrustal lithosphere (mantle lithosphere) as it descends into the mantle. We report on a series of scaled three‐dimensional (3‐D) analogue and 2‐D numerical experiments of coupled crust‐mantle dynamics that study such a process. The models investigate the effects of two geometries of RT instability (linear versus axisymmetric) and use different rheological stratifications of the crust. The growth of the RT instability is strongly influenced by crustal rheology. While RT growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself, the surface is. Experiments having a viscous‐only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions. In experiments with a brittle upper crust the surface deformation is much more subdued. However, strong mantle flow‐induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust. When the lower crust is relatively strong, localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability. In all the experiments, a portion of the ductile lower crustal material is entrained within the downgoing RT instability deep into the mantle. The interplay between dynamic topography and crustal thickening/thinning induced by the underlying mantle flow governs various time‐dependent phases of subsidence and uplift of the model surface. The experiments help to account for a number of first‐order tectonic behaviors of the lithospheres of Earth and other terrestrial planets. For example, intraplate orogens and basins, complex localized tectonic structure in intraplate settings, and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere RT instabilities.

Mantle plume heads and the initiation of plate tectonic reorganizations

We present results of numerical simulations which examine the plausibility of a reorganization of plate motions brought about by the interaction of a hot, low-viscosity plume head with the underside of a tectonic plate. A numerical model of highly viscous fluid flow driven by thermal buoyancy is employed to examine the interaction of mantle plumes with surface plates in a two-dimensional cartesian geometry. Plate-like boundary conditions applied to the upper surface consist of a large active plate with fixed speed plus a smaller passive plate whose speed and direction of motion are permitted to react to tractions induced by the underlying mantle flow (driven by the active plate). The passive plate is also subject to applied end-loads intended to simulate incipient subduction. The mantle plume is initiated with a hot patch on the lower boundary, and viscosity varies with temperature following an Arrhenius rheology. The calculations, performed with fixed aspect ratio (4 × 1) and fixed Rayleigh number (Ra = 10 6, where Ra refers to the strength of the starting plume), were found to depend on three parameters: the strength of the viscosity variation, the size of the applied end-load, and the speed of the active plate. These calculations indicate that plate motions can be sharply influenced by the lubricating effects of mantle plume heads even to the point of causing a reversal in the direction of plate motion.

Delamination and asymmetric lithospheric thickening in the development of the Tyrrhenian Rift

Summary Late tectonic extensional basins are superimposed on the continental collision suture at certain points along the Alpine belt. The onset of extension in these basins (Alboran, Tyrrhenian, Pannonian and Aegean) postdates the continental collision by only a few million years. In the case of the Tyrrhenian region, the rifting of the collision suture is accompanied by contemporaneous shortening in the Sicilian/southern Apennine thrust belt. We use finite element models to test whether the rapid evolution of the collision suture into a zone of rifting, and the development of closely juxtaposed regions of compression and extension, can be explained by a mechanism involving collision-induced delamination of continental mantle lithosphere. With delamination underway, the models predict the development of a region of contraction flanked by regions of extension, but no appreciable migration of the delamination front. Investigation of various initial lithospheric geometries shows that a uniformly thickened lithosphere will not readily delaminate, and that delamination is triggered if crustal and lithospheric thickening are offset. Asymmetric, or offset, lithospheric thickening produces an asymmetry in underlying mantle flow which gives rise to extension which is largely restricted to one side of the region of compression. The juxtaposition of extension and shortening in the Tyrrhenian/Calabrian region, as well as the greater degree of extension in the Tyrrhenian relative to the Ionian basin, may be a result of asymmetric lithospheric thickening in the initial collision suture.