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Sim, SJ, Stegman DR, Coltice N.  2016.  Influence of continental growth on mid-ocean ridge depth. Geochemistry Geophysics Geosystems. 17:4425-4437.   10.1002/2016gc006629   AbstractWebsite

The interconnectedness of life, water, and plate tectonics is strikingly apparent along mid-ocean ridges (MOR) where communities of organisms flourish off the disequilibrium of chemical potentials created by circulation of hydrothermal fluids driven by Earth's heat. Moreover, submarine hydrothermal environments may be critical for the emergence of life on Earth. Oceans were likely present in the Hadean but questions remain about early Earth's global tectonics, including when seafloor spreading began and whether mid-oceanic ridges were deep enough for maximum hydrothermal activities. For example, plate tectonics influences global sea level by driving secular variations in the volume of ocean basins due to continental growth. Similarly, variations in the distribution of seafloor age and associated subsidence, due to assembly and dispersal of supercontinents, explain the largest sea level variation over the past 140 Myr. Using synthetic plate configurations derived from numerical models of mantle convection appropriate for early Earth, we show that MOR have remained submerged and their depths potentially constant over geologic time. Thus, conditions in the early Earth existed for hydrothermal vents at similar depths as today, providing environments conducive for the development of life and allowing for processes such as hydrothermal alteration of oceanic crust to influence the mantle's geochemical evolution.

Petersen, RI, Stegman DR, Tackley PJ.  2015.  A regime diagram of mobile lid convection with plate-like behavior. Physics of the Earth and Planetary Interiors. 241:65-76.   10.1016/j.pepi.2015.01.002   AbstractWebsite

Terrestrial planetary bodies that undergo solid-state convection can exhibit a variety of tectonic styles, from stagnant lid one-plate planets to those with a mobilized lithosphere. For modeled planetary convection the de facto mode of recycling of lithosphere into the planetary interior is typically achieved through 2-sided and symmetric downwelling flows. However, lithosphere recycling on Earth occurs in a distinctly 1-sided mode known as subduction. Using numerical models of mantle convection in which the viscosity of planetary mantle material is strongly temperature-dependent, yet maintains a finite material strength as dictated by its yield stress, we investigate the continuum of mobile lid convection with plate-like behavior. The models span a parameter space of Rayleigh number and plate strength, and explore convective systems with low yield stresses resulting in weak subduction hinges that bend easily and highly deformable subducted slabs. Three distinct modes are found to occur in convective systems with weak plates: the stagnant lid mode, 2-sided downwelling mode, and a mode that alternates between 1-sided subduction and 2-sided downwellings. We classify the style of convective downwelling for a range of models and show that mode selection strongly depends on the combination of surface mobility and strength of the downgoing plate. Using these measurements, we have developed a regime diagram that can predict whether a particular system will be in one of those three modes. (C) 2015 Elsevier B.V. All rights reserved.

Druken, KA, Kincaid C, Griffiths RW, Stegman DR, Hart SR.  2014.  Plume-slab interaction: The Samoa-Tonga system. Physics of the Earth and Planetary Interiors. 232:1-14.   10.1016/j.pepi.2014.03.003   AbstractWebsite

Mantle plume behavior near subducting plates is still poorly understood and in fact varies significantly from the classical hotspot model. We investigate using 3D laboratory models how subduction-driven flow relates to the deformation and dispersal of a nearby plume. Results show slab-driven flow severely distorts plume-driven flow, entraining and passively advecting plume material despite its thermal buoyancy. Downdip sinking of the slab initially stalls vertical plume ascent while the combination of downdip and rollback sinking motions redistribute material throughout the system. As a consequence of the subduction-induced flow, surface expressions differ significantly from traditional plume expectations. Variations in slab sinking style and plume position lead to a range in head and conduit melting signatures, as well as migrating hotspots. For the Samoa-Tonga system, model predictions are consistent with proposed entrainment of plume material around the subducting plate. (C) 2014 Elsevier B.V. All rights reserved.

Davies, CJ, Stegman DR, Dumberry M.  2014.  The strength of gravitational core-mantle coupling. Geophysical Research Letters. 41:3786-3792.   10.1002/2014gl059836   AbstractWebsite

Gravitational coupling between Earth's core and mantle has been proposed as an explanation for a 6 year variation in the length-of-day (Delta LOD) signal and plays a key role in the possible superrotation of the inner core. Explaining the observations requires that the strength of the coupling, Gamma, falls within fairly restrictive bounds; however, the value of Gamma is highly uncertain because it depends on the distribution of mass anomalies in the mantle. We estimate Gamma from a broad range of viscous mantle flow models with density anomalies inferred from seismic tomography. Requiring models to give a correlation larger than 70% to the surface geoid and match the dynamic core-mantle boundary ellipticity inferred from Earth's nutations, we find that 3 x 10(19) < Gamma < 2 x 10(20) N m, too small to explain the 6 year Delta LOD signal. This new constraint on Gamma has important implications for core-mantle angular momentum transfer and on the preferred mode of inner core convection.

Jackson, CRM, Ziegler LB, Zhang HL, Jackson MG, Stegman DR.  2014.  A geochemical evaluation of potential magma ocean dynamics using a parameterized model for perovskite crystallization. Earth and Planetary Science Letters. 392:154-165.   10.1016/j.epsl.2014.01.028   AbstractWebsite

Magnesium perovskite (MgPv) is likely the first phase to crystallize from a deep magma ocean. Consequently, MgPv crystallization has a strong control on the dynamics and chemical evolution associated with the earliest stages of silicate Earth differentiation. In order to better understand the chemical evolution associated with MgPv crystallization during a magma ocean, a parameterized model for major and trace element partitioning by MgPv has been developed. The parameterization is based on a compilation of published experimental data and is applied to batch and near-fractional crystallization scenarios of ultramafic liquids, allowing for a more complete analysis of the geochemical implications for magma ocean crystallization. The chemical signatures associated with modeled MgPv fractionation are evaluated in the context of possible dynamical outcomes to a magma ocean (e.g. basal magma ocean (BMO) or crystal settling). It is shown that fractionating MgPv from ultramafic liquids imparts diagnostic signatures (e.g. Ca/Al, HFSE anomalies, epsilon Hf-176-epsilon Nd-143) in both the liquid and solid phases. These signatures are not currently observed in the accessible Earth, suggesting that either early-fractionating MgPv was subsequently homogenized or crystal suspension was dominant during the earliest stages of magma ocean crystallization. A BMO that fractionates CaPv and MgPv is also considered and shown to mute many of unobserved geochemical effects associated with a MgPv-only fractionation, offering an alternative possibility for the evolution of a BMO depleted in heat producing elements. (C) 2014 Elsevier B.V. All rights reserved.

Kincaid, C, Druken KA, Griffiths RW, Stegman DR.  2013.  Bifurcation of the Yellowstone plume driven by subduction-induced mantle flow. Nature Geoscience. 6:395-399.   10.1038/ngeo1774   AbstractWebsite

The causes of volcanism in the northwestern United States over the past 20 million years are strongly contested(1). Three drivers have been proposed: melting associated with plate subduction(2,3); tectonic extension and magmatism resulting from rollback of a subducting slab(4-6); or the Yellowstone mantle plume(7-9). Observations of the opposing age progression of two neighbouring volcanic chains-the Snake River Plain and High Lava Plains-are often used to argue against a plume origin for the volcanism. Plumes are likely to occur near subduction zones(10), yet the influence of subduction on the surface expression of mantle plumes is poorly understood. Here we use experiments with a laboratory model to show that the patterns of volcanism in the northwestern United States can be explained by a plume upwelling through mantle that circulates in the wedge beneath a subduction zone. We find that the buoyant plume may be stalled, deformed and partially torn apart by mantle flow induced by the subducting plate. Using plausible model parameters, bifurcation of the plume can reproduce the primary volcanic features observed in the northwestern United States, in particular the opposite progression of two volcanic chains. Our results support the presence of the Yellowstone plume in the northwestern United States, and also highlight the power of plume-subduction interactions to modify surface geology at convergent plate margins.

Ziegler, LB, Stegman DR.  2013.  Implications of a long-lived basal magma ocean in generating Earth's ancient magnetic field. Geochemistry Geophysics Geosystems. 14:4735-4742.   10.1002/2013gc005001   AbstractWebsite

Observations of Earth's magnetic field extending back to 3.45 billion years ago indicate that generation by a core dynamo must be sustained over most of Earth's history. However, recent estimates of thermal and electrical conductivity of liquid iron at core conditions from mineral physics experiments indicate that adiabatic heat flux is approximately 15 TW, nearly three times larger than previously thought, exacerbating difficulties for driving a core dynamo throughout Earth history by convective core cooling alone. Here, we explore the geomagnetic consequences of a basal magma ocean layer in the lowermost mantle, hypothesized to exist in the early Earth and perhaps surviving until well after the Archean. While the modern, solid lower mantle is an electromagnetic insulator, electrical conductivities of silicate melts are known to be higher, though as yet they are unconstrained for lowermost mantle conditions. We consider a range of possible electrical conductivities and find that for the highest electrical conductivities considered, a long-lived basal magma ocean could be a primary dynamo source region. This would suggest the proposed three magnetic eras observed in paleomagnetic data originate from distinct sources for dynamo generation: from 4.5 to 2.45 Ga within a basal magma ocean, from 2.25 to 0.4 Ga within a superadiabatically cooled liquid core, and from 0.4 Ga to present within a quasi-adiabatic core that includes a solidifying inner core.

Liu, LJ, Stegman DR.  2012.  Origin of Columbia River flood basalt controlled by propagating rupture of the Farallon slab. Nature. 482:386-U1508.   10.1038/nature10749   AbstractWebsite

The origin of the Steens-Columbia River (SCR) flood basalts, which is presumed to be the onset of Yellowstone volcanism, has remained controversial, with the proposed conceptual models involving either a mantle plume(1-5) or back-arc processes(6-8). Recent tomographic inversions based on the US Array data reveal unprecedented detail of upper-mantle structures of the western USA(9) and tightly constrain geodynamic models simulating Farallon subduction, which has been proposed to influence the Yellowstone volcanism(5,6). Here we show that the best-fitting geodynamic model(10) depicts an episode of slab tearing about 17 million years ago under eastern Oregon, where an associated sub-slab asthenospheric upwelling thermally erodes the Farallon slab, leading to formation of a slab gap at shallow depth. Driven by a gradient of dynamic pressure, the tear ruptured quickly north and south and within about two million years covering a distance of around 900 kilometres along all of eastern Oregon and northern Nevada. This tear would be consistent with the occurrence of major volcanic dikes during the SCR-Northern Nevada Rift flood basalt event both in space and time. The model predicts a petrogenetic sequence for the flood basalt with sources of melt starting from the base of the slab, at first remelting oceanic lithosphere and then evolving upwards, ending with remelting of oceanic crust. Such a progression helps to reconcile the existing controversies on the interpretation of SCR geochemistry and the involvement of the putative Yellowstone plume. Our study suggests a new mechanism for the formation of large igneous provinces.

Schellart, WP, Stegman DR, Farrington RJ, Moresi L.  2011.  Influence of lateral slab edge distance on plate velocity, trench velocity, and subduction partitioning. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008535   AbstractWebsite

Subduction of oceanic lithosphere occurs through both trenchward subducting plate motion and trench retreat. We investigate how subducting plate velocity, trench velocity and the partitioning of these two velocity components vary for individual subduction zone segments as a function of proximity to the closest lateral slab edge (D(SE)). We present a global compilation for 207 trench segments from 17 active subduction zones on Earth and three-dimensional numerical models of progressive free subduction of a single oceanic plate that subducts into a stratified mantle. The results show that the subducting plate velocity is always high (<= 5.1 cm/yr (models) and >= 4.2 cm/yr (nature)) and trench velocity is always low (<= 2.5 cm/yr (models) and <= 1.7 cm/yr (nature)) in the center of wide subduction zones (D(SE) > 2200 km). Only in regions close to lateral slab edges (D(SE) < 1000 km), be it for narrow or wide subduction zones, can the trench velocity exceed 4 cm/yr (models) and 6 cm/yr (nature) and can the subducting plate velocity go below 4 cm/yr (models) and 2 cm/yr (nature). In general, plate velocities, trench velocities and subduction partitioning are much more variable near slab edges than in the center of wide subduction zones owing to other parameters that affect subduction kinematics. We conclude that subduction kinematics can vary considerably along individual subduction zones and that the upper bound values for trench velocity and lower bound values for subducting plate velocity and subduction partitioning at individual subduction zone segments depend critically on D(SE).

Liu, LJ, Stegman DR.  2011.  Segmentation of the Farallon slab. Earth and Planetary Science Letters. 311:1-10.   10.1016/j.epsl.2011.09.027   AbstractWebsite

Recent tomography images reveal a complex 3D mantle structure beneath western United States, with feature morphology varying rapidly with depth. By assimilating plate motion history, paleo-age of sea floor, and paleo-geography of plate boundaries in a 3-D numerical model, we simulate the Farallon-Juan de Fuca subduction during the past 40 Ma. We find that the highly segmented upper mantle structure of western U.S. is a direct result of the Farallon subduction. We show that the tilted 'horseshoe'-shaped fast seismic anomaly beneath Nevada and Utah at 300-600 km depth range is in fact a segment of curled slab subducted since 15 Ma, and the shallower linear slab beneath the Cascades is younger than 5 Ma. The distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench roll-back since 20 Ma, the northward migrating JF-PA-NA triple-junction, and the toroidal flow around slab edges. The observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape, depth, and location of modeled subducted slab segments. The inferred viscosity for the asthenosphere is 5 x 10(19) Pa s and those for the transition zone and lower mantle are 1.5 x 10(21) Pa s and 2 x 10(22) Pa s, respectively. The slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth, but of similar order of magnitude viscosity in the transition zone. (C) 2011 Elsevier B.V. All rights reserved.

Cande, SC, Stegman DR.  2011.  Indian and African plate motions driven by the push force of the Reunion plume head. Nature. 475:47-52.   10.1038/nature10174   AbstractWebsite

Mantle plumes are thought to play an important part in the Earth's tectonics, yet it has been difficult to isolate the effect that plumes have on plate motions. Here we analyse the plate motions involved in two apparently disparate events-the unusually rapid motion of India between 67 and 52 million years ago and a contemporaneous, transitory slowing of Africa's motion-and show that the events are coupled, with the common element being the position of the Indian and African plates relative to the location of the Reunion plume head. The synchroneity of these events suggests that they were both driven by the force of the Reunion plume head. The recognition of this plume force has substantial tectonic implications: the speed-up and slowdown of India, the possible cessation of convergence between Africa and Eurasia in the Palaeocene epoch and the enigmatic bends of the fracture zones on the Southwest Indian Ridge can all be attributed to the Reunion plume.

Capitanio, FA, Faccenna C, Zlotnik S, Stegman DR.  2011.  Subduction dynamics and the origin of Andean orogeny and the Bolivian orocline. Nature. 480:83-86.   10.1038/nature10596   AbstractWebsite

The building of the Andes results from the subduction of the oceanic Nazca plate underneath the South American continent(1,2). However, how and why the Andes and their curvature, the Bolivian orocline, formed in the Cenozoic era (65.5 million years (Myr) ago to present), despite subduction continuing since the Mesozoic era(3) (251.0-65.5 Myr ago), is still unknown. Three-dimensional numerical subduction models demonstrate that variations in slab thickness, arising from the Nazca plate's age at the trench, produce a cordilleran morphology consistent with that observed(1,2). The age-dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate, causing it to thicken. Thus, subducting older Nazca plate below the Central Andes can explain the locally thickened crust and higher elevations. Here we demonstrate that resultant thickening of the South American plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the Bolivian orocline. Additionally, the varying forcing along the margin allows stress belts to form in the upper-plate interior, explaining the widening of the Central Andes and the different tectonic styles found on their margins, the Eastern and Western Cordilleras(2). The rise of the Central Andes and orocline formation are directly related to the local increase of Nazca plate age and an age distribution along the margin similar to that found today; the onset of these conditions only occurred in the Eocene epoch(4). This may explain the enigmatic delay of the Andean orogeny, that is, the formation of the modern Andes.

Yanagisawa, T, Yamagishi Y, Hamano Y, Stegman DR.  2010.  Mechanism for generating stagnant slabs in 3-D spherical mantle convection models at Earth-like conditions. Physics of the Earth and Planetary Interiors. 183:341-352.   10.1016/j.pepi.2010.02.005   AbstractWebsite

Seismic tomography reveals the natural mode of convection in the Earth is whole mantle with subducted slabs clearly seen as continuous features into the lower mantle. However, simultaneously existing alongside these deep slabs are stagnant slabs which are, if only temporarily, trapped in the upper mantle. Previous numerical models of mantle convection have observed a range of behavior for slabs in the transition zone depending on viscosity stratification and mineral phase transitions, but typically only exhibit flat-lying slabs when mantle convection is layered or trench migration is imposed. We use 3-D spherical models of mantle convection which range up to Earth-like conditions in Rayleigh number to systematically investigate three effects on mantle dynamics: (1) the mineral phase transitions, (2) a strongly temperature-dependent viscosity with plastic yielding at shallow depth, and (3) a viscosity increase in the lower mantle. First a regime diagram is constructed for isoviscous models over a wide range of Rayleigh number and Clapeyron slope for which the convective mode is determined. It agrees very well with previous results from 2-D simulations by Christensen and Yuen (1985), suggesting present-day Earth is in the intermittent convection mode rather than layered or strictly whole mantle. Two calculations at Earth-like conditions (Ra and Ra(H) = 2 x 10(7) and 5 x 10(8), respectively) which include effects (2) and (3) are produced with and without the effect of the mineral phase transitions. The first calculation (without the phase transition) successfully produces plate-like behavior with a long wavelength structure and surface heat flow similar to Earth's value. While the observed convective flow pattern in the lower mantle is broader compared to isoviscous models, it basically shows the behavior of whole mantle convection, and does not exhibit any slab flattening at the viscosity increase at 660 km depth. The second calculation which includes the phase transitions successfully exhibits the coexisting state of stagnant and penetrating slabs within the transition zone. These results indicate the importance of both a viscosity increase and mineral phase transitions for generating the structure of stagnant slabs observed by seismic tomography. (C) 2010 Elsevier B.V. All rights reserved.

Stegman, DR, Schellart WP, Freeman J.  2010.  Competing influences of plate width and far-field boundary conditions on trench migration and morphology of subducted slabs in the upper mantle. Tectonophysics. 483:46-57.   10.1016/j.tecto.2009.08.026   AbstractWebsite

Seismic tomography of the mantle provides a snapshot view of the present-day structure of subducted tectonic plates (slabs) in the mantle. However, a correct interpretation of how such features developed requires knowledge of both the history of plate motions, in particular the locations of subduction zones over time, as well as an understanding of how slabs deform and interact with the lower mantle. The understanding of seismic tomography requires a detailed knowledge of the motion of the trench with time, as well as the time-evolution of subduction zones and their associated slabs. We use 3-D numerical models to investigate two effects which can strongly influence the kinematics of subduction, in particular the migration of the trench: the width of the plate and the far-field boundary conditions of the subducting plate. These models study the full range of plate widths (300-6000 km) and are subjected to the full range of boundary conditions ("fixed,""free," and "pushed") representative of the tectonic settings found on Earth. Additionally, our results indicate that trailing edge boundary conditions dominate over the effect of plate width. Each boundary condition is identified to produce a particular trench migration behavior with an associated upper mantle slab morphology: (1) fixed edges result in retreating trenches and retrograde motion of the slab which ends up lying flat atop the lower mantle, (2) free-trailing edges tend to exhibit quasi-stationary trenches with slabs developing into folded piles atop the lower mantle, and (3) pushed trailing edges generally result in advancing trenches that overrun previously subducted material to form forward-draping slabs lying prone atop the lower mantle. This range of behavior can be observed in seismic tomography and an example of each is provided. Within each suite of models with similar boundary conditions, plate width has a subsidiary role in the observed slab morphology but three distinct regimes of plate width can be distinguished in regards to trench migration. Thus plate width is an important effect on trench migration and does result in notable differences in slab morphologies between narrow and wide plate widths. (C) 2009 Elsevier B.V. All rights reserved.

Farrington, RJ, Stegman DR, Moresi LN, Sandiford M, May DA.  2010.  Interactions of 3D mantle flow and continental lithosphere near passive margins. Tectonophysics. 483:20-28.   10.1016/j.tecto.2009.10.008   AbstractWebsite

We investigate the time evolution of 3D numerical models of convection in the upper mantle which incorporate both plate motions and thick continental lithosphere. The resultant flow in the upper mantle is driven by a combination of bottom heated convection and applied shear velocity boundary conditions that represents plate motion. Both the plate velocity and continental lithosphere topography are varied in a way to assess the general influence of 3D geometry as well as a more specific tectonic analogue of the Australian plate. Transient thermal events offshore of the trailing passive margin are observed and include plume migration, boundary layer instability growth at the passive margin and variations in surface heat flux. The geometry and plate velocity both play a significant role in controlling the magnitude and duration of these transient features. In particular, there are large differences between the different models in the oceanic region downstream of the trailing edge of the continent At near-stationary plate speeds, cold linear downwelling sheets propagate away from the 3D edge of the continent, with regions offshore of the continents central axis localising hot cylindrical upwelling plumes. At very fast plate speeds, the shear flow is dominated by the plate motions. This causes regions neighbouring the trailing edge of the continent to produce broad, hot upwellings and the cold linear sheets to migrate around the continent. At moderate (2 cm/yr) plate speeds, oceanic lithosphere neighbouring the passive margin along the trailing edge of the continent is buffered by cold, downwelling instabilities sinking along the edges of the continental lithosphere. Such neighbouring regions are subjected to larger heat flux than for regions distant to the passive margin, yet also record smaller and less variable vertical surface velocities. These dynamics have implications for volcanism and surface topography, for which 3D aspects play a significant role. (C) 2009 Elsevier B.V. All rights reserved.

Stegman, DR, Farrington R, Capitanio FA, Schellart WP.  2010.  A regime diagram for subduction styles from 3-D numerical models of free subduction. Tectonophysics. 483:29-45.   10.1016/j.tecto.2009.08.041   AbstractWebsite

Previous models of subduction (both analogue and numerical) have observed a number of distinct styles of subduction, each with particular subduction motions (partitioned between slab rollback and forward plate advance) and associated slab morphologies. We use 3-D numerical models to investigate subduction dynamics by varying the strength of slabs as well as the buoyancy, and propose a new classification based on these parameters. The slab strength is specified both through the ratio of viscosities between the subducting plate and upper mantle (eta(plate)/eta(um)) as well as the plate thickness, hp,,,e. Only a very restrictive range of plates ("strong" plates with smaller buoyancy) tend to favor modes of subduction which are exclusively advancing. Plates which have greater negative buoyancy will eventually transition into a retreating style. We find that the flexural strength and the buoyancy determine the subduction style (as distinguished by a characteristic slab morphology), and control several subduction characteristics including the partitioning between slab rollback and plate advance, the trench curvature, and the slab's radius of curvature. Plates that are 80-100 km thick with eta(plate)/eta(um) similar to 100-300 are classified here as "weak" and are the only plates to exhibit slab geometry with several recumbent folds atop the more viscous lower mantle. This regime of weak plates with their associated slab morphologies (predominant folding) is argued to be most similar to slabs on Earth based on the presence of folded slab piles in Earth's upper mantle (as interpreted from seismic tomography). (C) 2009 Elsevier B.V. All rights reserved.

Capitanio, FA, Stegman DR, Moresi LN, Sharples W.  2010.  Upper plate controls on deep subduction, trench migrations and deformations at convergent margins. Tectonophysics. 483:80-92.   10.1016/j.tecto.2009.08.020   AbstractWebsite

Thus far, relatively simplistic models of free subduction, in which the trench and plate motions are emergent features completely driven by the negative buoyancy of the slab, have investigated the dynamics of a single, isolated subducting plate. Here we extend such models to incorporate an overriding plate and present the results of how such an overriding plate feedbacks into the dynamics of free subduction. In this study, we address three fundamental aspects of these dynamics: 1) How does the presence of an overriding plate change the force balance at the convergent margins? 2) How are the forces from deep subduction propagated to the surface? And 3) what controls the stress regime in a system of coupled upper and subducting plates and how is it expressed in the deformations and plate motions? In general, we find that the evolution of subduction zones is strongly controlled by both the interactions between the slab and the upper-lower mantle discontinuity as well as the strength of the upper plate. When either the subducting or upper plates are unable to move, subduction motions are steady state and partitioned entirely into either slab rollback or plate advance, respectively. When conditions favour a quasi-stationary trench, the subducted lithosphere can form into a pile with multiple recumbent folds of slab material atop the lower mantle. Alternating between forwards- and backwards-draping slab, the corresponding horizontal trench motions at the surface are frontward and rearward, respectively, resulting in either a compressive or extensional regime in the back-arc. Time-dependent forcing arising from the slab piling behaviour can have a feedback with upper plate and produce strongly non-steady state, intermittent phases of upper plate deformation as those commonly observed on Earth. Two types of discontinuous back-arc strain evolution are identified: (1) periodic, when recurrent phases of strain over finite durations are accommodated by (viscous) stretching/thickening of the plate, and (2) episodic, when upper plate deformation localizes (plastic strain) and allows for punctuated episodes. These phases can include extension, quiescence, and compression, giving rise to a large variety of possible tectonic evolutions. The models presented here provide insight into the dynamics behind the non-steady-state evolution of subduction, which can help unravel seemingly erratic motions of major convergent margins and back-arc deformations around the Pacific and Indian Oceans during the Cenozoic. (C) 2009 Elsevier BM. All rights reserved.

Schellart, WP, Stegman DR, Farrington RJ, Freeman J, Moresi L.  2010.  Cenozoic Tectonics of Western North America Controlled by Evolving Width of Farallon Slab. Science. 329:316-319.   10.1126/science.1190366   AbstractWebsite

Subduction of oceanic lithosphere occurs through two modes: subducting plate motion and trench migration. Using a global subduction zone data set and three-dimensional numerical subduction models, we show that slab width (W) controls these modes and the partitioning of subduction between them. Subducting plate velocity scales with W(2/3), whereas trench velocity scales with 1/W. These findings explain the Cenozoic slowdown of the Farallon plate and the decrease in subduction partitioning by its decreasing slab width. The change from Sevier-Laramide orogenesis to Basin and Range extension in North America is also explained by slab width; shortening occurred during wide-slab subduction and overriding-plate-driven trench retreat, whereas extension occurred during intermediate to narrow-slab subduction and slab-driven trench retreat.

Stegman, DR, Freeman J, May DA.  2009.  Origin of ice diapirism, true polar wander, subsurface ocean, and tiger stripes of Enceladus driven by compositional convection. Icarus. 202:669-680.   10.1016/j.icarus.2009.03.017   AbstractWebsite

We consider the scenario in which the presence of ammonia in the bulk composition of Enceladus plays a pivotal role in its thermochemical evolution. Because ammonia reduces the melting temperature of the ice shell by 100 K below that of pure water ice, small amounts of tidal dissipation can power an "ammonia feedback" mechanism that leads to secondary differentiation of Enceladus within the ice shell. This leads to compositionally distinct zones at the base of the ice shell arranged such that a layer of lower density (and compositionally buoyant) pure water ice underlies the undifferentiated ammonia-dihydrate ice layer-above. We then consider a large scale instability arising from the Pure water ice layer, and use a numerical model to explore the dynamics of compositional convection within the ice shell of Encelaclus. The instability of the layer can easily account for a diapir that is hemispherical in scale. As it rises to the surface, it co-advects the warm internal temperatures towards the outer layers of the satellite. This advected heat facilitates the generation of a subsurface ocean within the ice shell of Enceladus. This scenario can simultaneously account for the origin of asymmetry in surface deformation observed on Enceladus as well as two global features inferred to exist: a large density anomaly within the interior and a subsurface ocean underneath the south polar region. (C) 2009 Elsevier Inc. All rights reserved.

Gottschaldt, K. U. Walzer, SDRJR.  2009.  Mantle Dynamics-A case study. Advances in geocomputing. 119( Xing H, Ed.).:139-170., Heidelberg: Springer Verlag Berlin   10.1007/978-3-540-85879-9_5   Abstract

Solid state convection in the rocky mantles is a key to understanding the thermochemical evolution and tectonics of terrestrial planets and moons. It is driven by internal heat and can be described by a system of coupled partial differential equations. There are no analytic solutions for realistic configurations and numerical models are an indispensable tool for researching mantle convection. After a brief general introduction, we introduce the basic equations that govern mantle convection and discuss some common approximations. The following case study is a contribution towards a self-consistent thermochemical evolution model of the Earth. A crude approximation for crustal differentiation is coupled to numerical models of global mantle convection, focussing on geometrical effects and the influence of rheology on stirring. We review Earth-specific geochemical and geophysical constraints, proposals for their reconciliation, and discuss the implications of our models for scenarios of the Earth’s evolution. Specific aspects of this study include the use of passive Lagrangian tracers, highly variable viscosity in 3-d spherical geometry, phase boundaries in the mantle and a parameterised model of the core as boundary condition at the bottom of the mantle.

Schellart, WP, Stegman DR, Freeman J.  2008.  Global trench migration velocities and slab migration induced upper mantle volume fluxes: Constraints to find an Earth reference frame based on minimizing viscous dissipation. Earth-Science Reviews. 88:118-144.   10.1016/j.earscirev.2008.01.005   AbstractWebsite

Since the advent of plate tectonics different global reference frames have been used to describe the motion of plates and trenches. The difference in plate motion and trench migration between different reference frames can be substantial (up to 4 cm/yr). This study presents an overview of trench migration velocities for all the mature and incipient subduction zones on Earth as calculated in eight different global reference frames. Calculations show that, irrespective of the reference frame: (1) trench retreat always dominates over trench advance, with 62-78% of the 244 trench segments retreating; (2) the mean and median trench velocity are always positive (retreating) and within the range 1.3-1.5 cm/yr and 0.9-1.3 cm/yr, respectively; (3) rapid trench retreat is only observed close to lateral slab edges (< 1500 km); and (4) trench retreat is always slow far from slab edges (> 2000 km). These calculations are predicted by geodynamic models with a varying slab width, in which plate motion, trench motion and mantle flow result from subduction of dense slabs, suggesting that trench motion is indeed primarily driven by slab buoyancy forces and that proximity to a lateral slab edge exerts a dominant control on the trench migration velocity. Despite these four general conclusions, significant differences in velocities between such reference frames remain. It is therefore important to determine which reference frame most likely describes the true absolute velocities to get an understanding of the forces driving plate tectonics and mantle convection. It is here proposed that, based on fluid dynamic considerations and predictions from geodynamic modelling, the best candidate is the one, which optimises the number of trench segments that retreat, minimizes the trench-perpendicular trench migration velocity (v(T perpendicular to)) in the centre of wide (> 4000 km) subduction zones, maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge, minimizes the average of the absolute of the trench-perpendicular trench migration velocity (vertical bar vT perpendicular to vertical bar) for all subduction zones on Earth, and minimizes the global upper mantle toroidal volume flux (phi T-o) that results from trench migration and associated lateral slab migration (i.e. slab rollback or slab roll-forward). Calculations show that these conditions are best met in one particular Indo-Atlantic hotspot reference frame, where 75% of the subduction zones retreat, V-T perpendicular to in the centre of wide subduction zones ranges between -3.5 and 1.8 cm/yr, 83% of the trench segments located within 2000 km. of the closest lateral slab edge retreat, the average of vertical bar V-T perpendicular to vertical bar is 2.1 cm/yr, and phi T-o =456 km(3)/yr (lower limit) and 539 km(3)/yr (upper limit). Inclusion of all the incipient subduction zones on Earth results in slightly greater fluxes of 465 km(3)/yr (lower limit) and 569 km(3)/yr (upper limit). It is also found that this reference frame is close to minimizing the total sub-lithospheric upper mantle volume flux (phi(K)) associated with motion of continental keels located below the major cratons. It is stressed, however, that phi(K) is an order of magnitude smaller than phi T-o, and thus of subordinate importance. In conclusion, the Indo-Atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities. (c) 2008 Elsevier B.V. All rights reserved.

Clark, SR, Stegman D, Muller RD.  2008.  Episodicity in back-arc tectonic regimes. Physics of the Earth and Planetary Interiors. 171:265-279.   10.1016/j.pepi.2008.04.012   AbstractWebsite

The evolution of back-arc basins is tied to the development of the dynamics of the subduction system they are a part of. We present a study of back-arc basins and model their development by implementing 3D time-dependant computer models of subduction including an overriding plate. We define three types of episodicity: pseudo-, quasi- and hyper-episodicity, and find evidence of these in nature. Observations of back-arc basin ages, histories of spreading, quiescence and compression in the overriding plate give us an understanding of the time-development of these subduction zones and back-arc basins. Across the globe today, a number of trenches are advancing-the lzu-Bonin Trench, the Mariana Trench, the Japan Trench, the Java-Sunda Trench and the central portion of the Peru-Chile Trench (the Andes subduction zone). The lzu-Bonin, Mariana and Japan all have established back-arc basins, while the others have documented episodes of spreading, quiescence, compression or a combination of these. The combination of advancing and retreating trench motion places these subduction zones in the category of hyper-episodicity. Quasi-episodicity, in which the back-arc shifts between phases of rifting, spreading and quiescence, is the dominant form of episodic back-arc development in the present. We find this type of episodicity in models for which the system is dynamically consistent-that we have allowed the subducting plate's velocity to be determined by the sinking slabs' buoyancy. Quasi- and hyper-episodicity are only found in subduction zones with relatively high subducting plate velocities, between 6 and 9 cm/year. Finally, those subduction zones for which the subducting plate is moving slowly, such as in the Mediterranean or the Scotia Sea, experience only pseudo-episodicity, where the spreading moves linearly towards the trench but often does so in discrete ridge-jump events. (C) 2008 Elsevier B.V. All rights reserved.

OzBench, M, Regenauer-Lieb K, Stegman DR, Morra G, Farrington R, Hale A, May DA, Freeman J, Bourgouin L, Muhlhaus H, Moresi L.  2008.  A model comparison study of large-scale mantle-lithosphere dynamics driven by subduction. Physics of the Earth and Planetary Interiors. 171:224-234.   10.1016/j.pepi.2008.08.011   AbstractWebsite

Modelling subduction involves solving the dynamic interaction between a rigid (solid yet deformable) plate and the fluid (easily deformable) mantle. Previous approaches neglected the solid-like behavior of the lithosphere by only considering a purely fluid description. However, over the past 5 years, a more self-consistent description of a mechanically differentiated subducting plate has emerged. The key feature in this mechanical description is incorporation of a strong core which provides small resistance to plate bending at subduction zones while simultaneously providing adequate stretching resistance Such that slab Pull drives forward plate motion. Additionally, the accompanying numerical approaches for simulating large-scale lithospheric deformation processes coupled to the underlying viscous mantle flow, have been become available. Here we put forward three fundamentally different numerical strategies, each of which is capabable of treating the advection of mechanically distinct materials that describe the subducting plate. We demonstrate their robustness by calculating the numerically challenging problem of subduction of a 6000 kin wide slab at high-resolution in three-dimensions, the successfuly achievement of which only a few codes in the world can presently even attempt. In spite of the differences of the approaches, all three codes pass the simple qualitative test of developing an "S-bend" trench curvature previously observed in similar models. While reproducing this emergent feature validates that the lithosphere-mantle interaction has been correctly modelled, this is not a numerical benchmark in the traditional sense where the objective is for all codes to achieve exact agreement on a unique numerical Solution. However, we do provide some quantitative comparisons such as trench and plate kinematics in addition to discussing the strength and weaknesses of the individual approaches. Consequently, we believe these developed algorithms can now be applied to study the parameters involved in the dynamics of subduction and offer a toolbox to be used by the entire geoscience community. (C) 2008 Elsevier B.V. All rights reserved.

Stegman, DR, Moresi L, Turnbull R, Giordani J, Sunter P, Lo A, Quenette S.  2008.  gLucifer: next generation visualization framework for high-performance computational geodynamics. Visual Geosciences. 13:71-84.: Springer-Verlag   10.1007/s10069-008-0010-2   AbstractWebsite

High-performance computing provides unprecedented capabilities to produce higher resolution 4-D models in a fraction of time. Thus, the need exists for a new generation of visualization systems able to maintain parity with the enormous volume of data generated. In attempting to write this much data to disk, each computational step introduces a significant performance bottleneck, yet most existing visualization software packages inherently rely on reading data in from a dump file. Available packages make this assumption of postprocessing at quite a fundamental level and are not very well suited for plotting very large numbers of specialized particles. This necessitates the creation of a new visualization system that meets the needs of large-scale geodynamic modeling. We have developed such a system, gLucifer, using a software framework approach that allows efficient reuse of our efforts in other areas of research. gLucifer is capable of producing movies of a 4-D data set “on the fly” (simultaneously with running the parallel scientific application) without creating a performance bottleneck. By eliminating most of the human efforts involved in visualizing results through postprocessing, gLucifer reconnects the scientist to the numerical experiment as it unfolds. Data sets that were previously very difficult to even manage may be efficiently explored and interrogated without writing to disk, and because this approach is based entirely on memory distributed across as many processors as are being utilized by the scientific application, the visualization solution is scalable into terabytes of data being rendered in real time.

Goyette, S, Takatsuka M, Clark S, Müller DR, Rey P, Stegman DR.  2008.  Increasing the usability and accessibility of geodynamic modelling tools to the geoscience community: UnderworldGUI. Visual Geosciences. 13:25-36.: Springer-Verlag   10.1007/s10069-007-0006-3   AbstractWebsite

Geoscientists are faced with a number of complexities that represent obstacles to the development of realistic simulation of deep earth processes. Realistic 4D thermo-mechanical simulation using software packages like Underworld and Gale, when combined appropriately with geoscientific expertise, can lead to novel insights into the deformation of geological structures at a wide range of time and spatial scales. The challenge for end-user geoscientists lies in applying their knowledge within the framework of the software’s input specification, including initial, internal, and boundary conditions and output visualization parameters. We have built a Graphical User Interface (GUI) to remove many of the difficulties related to editing the Extensible Markup Language (XML) encoded input files of Underworld/Gale geomodels and therefore, to greatly broaden the user base of these software packages. By helping Underworld/Gale to meet a large audience, we provide a tool to the geoscience community that helps to move from untested conceptual models to physically valid, properly scaled modelling. Furthermore, the UnderworldGUI offers a mechanism for storing and retrieving experimental models in a centralised database, thus providing the geoscience community with a means to share the outcomes of its experimental research. Further details of the UnderworldGUI are available at the web site