Clinton Phillips Conrad

• Karlsen, Krister Stræte; Conrad, Clinton Phillips & Magni, Valentina (2019). Deep water cycling and sea level change since the breakup of Pangea. Geochemistry Geophysics Geosystems.  ISSN 1525-2027.  20(6), s 2919- 2935 . doi: 10.1029/2019GC008232 Show summary

  First‐order variations in sea level exhibit amplitudes of ∼200 m over periods that coincide with those of supercontinental cycles (∼300–500 Myr). Proposed mechanisms for this sea level change include processes that change the container volume of the ocean basins and the relative elevation of continents. Here we investigate how unbalanced rates of water exchange between Earth's surface and mantle interior, resulting from fluctuations in tectonic rates, can cause sea level changes. Previous modeling studies of subduction water fluxes suggest that the amount of water that reaches sub‐arc depths is well correlated with the velocity and age of the subducting plate. We use these models to calibrate a parameterization of the deep subduction water flux, which we together with a parameterization of mid‐ocean ridge outgassing, then apply to reconstructions of Earth's tectonic history. This allows us to estimate the global water fluxes between the oceans and mantle for the past 230 Myr and compute the associated sea level change. Our model suggests that a sea level drop of up to 130 m is possible over this period and that it was partly caused by the ∼150Ma rift pulse that opened the Atlantic and forced rapid subduction of old oceanic lithosphere. This indicates that deep water cycling may be one of the more important sea level changing mechanisms on supercontinental time scales and provides a more complete picture of the dynamic interplay between tectonics and sea level change.

• Paul, Jyotirmoy; Ghosh, Attreyee & Conrad, Clinton Phillips (2019). Traction and strain-rate at the base of the lithosphere: An insight into cratonic survival. Geophysical Journal International.  ISSN 0956-540X.  217(2), s 1024- 1033 . doi: 10.1093/gji/ggz079 Show summary

  Cratons are the oldest parts of the lithosphere, some of them surviving since Archean. Their long-term survival has sometimes been attributed to high viscosity and low density. In our study, we use a numerical model to examine how shear tractions exerted by mantle convection work to deform cratons by convective shearing. We find that although tractions at the base of the lithosphere increase with increasing lithosphere thickness, the associated strain rates decrease. This inverse relationship between stress and strain-rate results from lateral viscosity variations along with the model’s free slip condition imposed at the Earth’s surface, which enables strain to accumulate along weak zones at plate boundaries. Additionally, we show that resistance to lithosphere deformation by means of convective shearing, which we express as an apparent viscosity, scales with the square of lithosphere thickness. This suggests that the enhanced thickness of the cratons protects them from convective shear, and allows them to survive as the least deformed areas of the lithosphere. Indeed, we show that the combination of a smaller asthenospheric viscosity drop and a larger cratonic viscosity, together with the excess thickness of cratons compared to the surrounding lithosphere, can explain their survival since Archean time.

• Steinberger, Bernhard; Conrad, Clinton Phillips; Osei Tutu, Anthony & Hoggard, Mark J. (2019). On the amplitude of dynamic topography at spherical harmonic degree two. Tectonophysics.  ISSN 0040-1951.  760, s 221- 228 . doi: 10.1016/j.tecto.2017.11.032 Show summary

  Two large, seismically slow regions in the lower mantle beneath Africa and the Pacific Ocean are sometimes referred to as “superplumes”. This name evokes images of large-scale active upwellings. However, it remains unclear whether these features are real or represent collections of multiple regular mantle plumes. Here, we investigate the implications of these upwellings for dynamic topography. We combine detailed measurements of oceanic residual topography from Hoggard et al. (2016) with continental constraints derived from CRUST1.0 to produce a global model expanded in spherical harmonics. Observed dynamic topography is subsequently compared to predictions derived from mantle flow following Steinberger (2016) using tomographic density models. Results yield relatively good overall agreement and amplitude spectra with similar slopes, except for degree two (i.e. > 10,000 km wavelengths) where predicted amplitude is more than two times as large and is dominated by contributions from the lower mantle. Predictive models suggest two large-scale uplifted regions above the “superplumes” that are barely seen in the observed topography. We suggest that this mismatch can only partly be reconciled by altering the seismic velocity to density conversion factor or by including the effects of lower mantle chemical heterogeneity. In addition, it may be important to consider more significant revisions to the lower mantle flow patterns, such as those possibly induced by different radial viscosity profiles and laterally-varying or anisotropic lower mantle viscosity.

• Torsvik, Trond Helge; Steinberger, Bernhard; Shephard, Grace; Doubrovine, Pavel; Gaina, Carmen; Domeier, Mathew; Conrad, Clinton Phillips & Sager, William (2019). Our planet is a complicated dynamic system that is constantly evolving on a continuum of scales. On the large scale, the geography of Earth changes over millions of years due to the slow but constant motion of the tectonic plates and the convection of the mantle beneath them. On very long timescales, plate tectonics is a fascinatingly dramatic phenomenon that assembles and dissects continents, opens and closes oceans, and constructs and dismantles mountains. Unsurprisingly, these processes have. Geochemistry Geophysics Geosystems.  ISSN 1525-2027.  20(7), s 3659- 3689 . doi: 10.1029/2019GC008402
• Crameri, Fabio; Conrad, Clinton Phillips; Montési, Laurent & Lithgow-Bertelloni, Carolina (2018). The dynamic life of an oceanic plate. Tectonophysics.  ISSN 0040-1951.  760, s 107- 135 . doi: 10.1016/j.tecto.2018.03.016 Show summary

  As the Earth's primary mode of planetary cooling, the oceanic plate is created at mid-ocean ridges, transported across the planet's surface, and destroyed at subduction zones. The evolution of its buoyancy and rheology during its lifespan maintains the coherence of the plate as a distinct geological entity and controls the localised deformation and vertical material exchange at plate boundaries, which enables the horizontal ocean-plate movements. These motions intimately link the oceanic plate to the overarching overturn of Earth's mantle: The plate forms out of rising mantle material at spreading ridges; it cools the Earth's interior as the cold thermal boundary layer to mantle convection; and its sinking portions drive not only the plate itself but also dominate global flow in the mantle. We scrutinise here the entire life cycle of the oceanic plate, starting with its birth at the mid-ocean ridge, including the thermal, rheological, and chemical conditions of initiation, followed by plate maturation as it ages and cools while crossing the seafloor, and finishing with the dynamics of plate destruction as it retires at the subduction zone to become a deeper part of Earth's convective system. We find that the full range of dynamic behaviour of the oceanic plate, including its forcing and overall framework within Earth's convecting system, is not fully captured by the existing concept of Plate Tectonics, which describes solely the horizontal surface kinematics of all plates. Therefore, we introduce a more specific and at the same time more integral concept named “Ocean-Plate Tectonics” that more specifically describes the dynamic life of the oceanic plate and accounts for the knowledge gained during the past 50 years. This “Ocean-Plate Tectonics” must have emerged on Earth at least 1 Billion years ago, and dominates Earth's dynamics today.

• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2018). Stabilizing effect of compositional viscosity contrasts on thermochemical piles. Geophysical Research Letters.  ISSN 0094-8276.  45(15), s 7523- 7532 . doi: 10.1029/2018GL078799 Show summary

  The large low shear velocity provinces (LLSVPs) observed in the lowermost mantle are widely accepted as chemically distinct thermochemical 'piles', but their origin and long-term evolution remain poorly understood. The survival time and shape of the LLSVPs are thought to be mainly controlled by their compositional density, while their viscosity has beeen considered less important. Based on recent constraints on chemical reactions between mantle and core, a more complex viscosity structure of the lowermost mantle, possibly including high viscosity thermochemical pile material, seems reasonable. In this study, we use numerical models to identify a trade-off between compositional viscosity and density contrasts required for long-term stability of thermochemical piles, which permits lower-density and higher-viscosity piles. Moreover, our results indicate more restrictive stability conditions during periods of strong deformation-induced entrainment, e.g. during initial pile formation, which suggests long-term pile survival.

• Watkins, C. Evan & Conrad, Clinton Phillips (2018). Constraints on dynamic topography from asymmetric subsidence of the mid-ocean ridges. Earth and Planetary Science Letters.  ISSN 0012-821X.  484, s 264- 275 . doi: 10.1016/j.epsl.2017.12.028 Show summary

  Stresses from mantle convection deflect Earth's surface vertically, producing dynamic topography that is important for continental dynamics and sea-level change but difficult to observe due to overprinting by isostatic topography. For long wavelengths (∼104 km), the amplitude of dynamic topography is particularly uncertain, with mantle flow models typically suggesting larger amplitudes (>1000 m) than direct observations. Here we develop a new constraint on the amplitude of long-wavelength dynamic topography by examining asymmetries in seafloor bathymetry across mid-ocean ridges. We compare bathymetric profiles across the Mid-Atlantic Ridge (MAR) and the East Pacific Rise (EPR) and we find that the South American flank of both ridges subsides faster than its opposing flank. This pattern is consistent with dynamic subsidence across South America, supported by downwelling in the lower mantle. To constrain the amplitude of dynamic topography, we compare bathymetric profiles across both ridges after correcting bathymetry for several different models of dynamic topography with varying amplitudes and spatial patterns. We find that long-wavelength dynamic topography with an amplitude of only ∼500 m explains the observed asymmetry of the MAR. A similar model can explain EPR asymmetry but is complicated by additional asymmetrical topography associated with tectonic, crustal thickness, and/or asthenospheric temperature asymmetries across the EPR. After removing 500 m of dynamic topography, both the MAR and EPR exhibit a slower seafloor subsidence rate (∼280–290 m/Myr1/2) than previously reported. Our finding of only ∼500 m of long-wavelength dynamic topography may indicate the importance of thermochemical convection and/or large viscosity variations for lower mantle dynamics.

• Conrad, Clinton Phillips; Selway, K.; Hirschmann, M.M.; Ballmer, M.D. & Wessel, P. (2017). Constraints on volumes and patterns of asthenospheric melt from the space-time distribution of seamounts. Geophysical Research Letters.  ISSN 0094-8276.  44(14), s 7203- 7210 . doi: 10.1002/2017GL074098 Full text in Research Archive. Show summary

  Although partial melt in the asthenosphere is important geodynamically, geophysical constraints on its abundance remain ambiguous. We use a database of seamounts detected using satellite altimetry to constrain the temporal history of erupted asthenospheric melt. We find that intraplate volcanism on young seafloor (<60 Ma) equates to a ~20 m thick layer spread across the seafloor. If these seamounts tap partial melt within a ~20 km thick layer beneath the ridge flanks, they indicate extraction of an average melt fraction of ~0.1%. If they source thinner layers or more laterally restricted domains, larger melt fractions are required. Increased seamount volumes for older lithosphere suggest either more active ridge flank volcanism during the Cretaceous or additional recent melt eruption on older seafloor. Pacific basin age constraints suggest that both processes are important. Our results indicate that small volumes of partial melt may be prevalent in the upper asthenosphere across ocean basins.

• Dangendorf, Sönke; Marcos, Marta; Guy, Wöppelmann; Conrad, Clinton Phillips; Frederikse, Thomas & Riva, Riccardo (2017). Reassessment of 20th century global mean sea level rise. Proceedings of the National Academy of Sciences of the United States of America.  ISSN 0027-8424.  114(23), s 5946- 5951 . doi: 10.1073/pnas.1616007114 Full text in Research Archive. Show summary

  The rate at which global mean sea level (GMSL) rose during the 20th century is uncertain, with little consensus between various reconstructions that indicate rates of rise ranging from 1.3 to 2 mm⋅y−1. Here we present a 20th-century GMSL reconstruction computed using an area-weighting technique for averaging tide gauge records that both incorporates up-to-date observations of vertical land motion (VLM) and corrections for local geoid changes resulting from ice melting and terrestrial freshwater storage and allows for the identification of possible differences compared with earlier attempts. Our reconstructed GMSL trend of 1.1 ± 0.3 mm⋅y−1 (1σ) before 1990 falls below previous estimates, whereas our estimate of 3.1 ± 1.4 mm⋅y−1 from 1993 to 2012 is consistent with independent estimates from satellite altimetry, leading to overall acceleration larger than previously suggested. This feature is geographically dominated by the Indian Ocean–Southern Pacific region, marking a transition from lower-than-average rates before 1990 toward unprecedented high rates in recent decades. We demonstrate that VLM corrections, area weighting, and our use of a common reference datum for tide gauges may explain the lower rates compared with earlier GMSL estimates in approximately equal proportion. The trends and multidecadal variability of our GMSL curve also compare well to the sum of individual contributions obtained from historical outputs of the Coupled Model Intercomparison Project Phase 5. This, in turn, increases our confidence in process-based projections presented in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.

• Hansen, Lars N.; Conrad, Clinton Phillips; Boneh, Yuval; Skemer, Philip; Warren, Jessica M. & Kohlstedt, David L. (2016). Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model. Journal of Geophysical Research (JGR): Solid Earth.  ISSN 2169-9313.  121(10), s 7137- 7160 . doi: 10.1002/2016JB013240 Show summary

  The significant viscous anisotropy that results from crystallographic alignment (texture) of olivine grains in deformed upper mantle rocks strongly influences a large variety of geodynamic processes. Our ability to explore the effects of anisotropic viscosity in simulations of these processes requires a mechanical model that can predict the magnitude of anisotropy and its evolution. Unfortunately, existing models of olivine textural evolution and viscous anisotropy are calibrated for relatively small deformations and simple strain paths, making them less general than desired for many large-scale geodynamic scenarios. Here we develop a new set of micromechanical models to describe the mechanical behavior and textural evolution of olivine through a large range of strains and complex strain histories. For the mechanical behavior, we explore two extreme scenarios, one in which each grain experiences the same stress tensor (Sachs model) and one in which each grain undergoes a strain rate as close as possible to the macroscopic strain rate (pseudo-Taylor model). For the textural evolution, we develop a new model in which the director method is used to control the rate of grain rotation and the available slip systems in olivine are used to control the axis of rotation. Only recently has enough laboratory data on the deformation of olivine become available to calibrate these models. We use these new data to conduct inversions for the best parameters to characterize both the mechanical and textural evolution models. These inversions demonstrate that the calibrated pseudo-Taylor model best reproduces the mechanical observations. Additionally, the pseudo-Taylor textural evolution model can reasonably reproduce the observed texture strength, shape, and orientation after large and complex deformations. A quantitative comparison between our calibrated models and previously published models reveals that our new models excel in predicting the magnitude of viscous anisotropy and the details of the textural evolution. In addition, we demonstrate that the mechanical and textural evolution models can be coupled and used to reproduce mechanical evolution during large-strain torsion tests. This set of models therefore provides a new geodynamic tool for incorporating viscous anisotropy into large-scale numerical simulations.

• Plyusnina, Ekaterina E.; Ruban, Dmitry A.; Conrad, Clinton Phillips; Zerfass, Geise de Santana dos Anjos & Zerfass, Henrique (2016). Long-term eustatic cyclicity in the Paleogene: a critical assessment. Proceedings Geological Association.  ISSN 0016-7878.  127(4), s 425- 434 . doi: 10.1016/j.pgeola.2016.03.006 Full text in Research Archive. Show summary

  Global sea level has changed cyclically throughout Earth's history due to a variety of mechanisms that operate on a variety of timescales. Here we attempt to place constraints on the “actual” number of sea-level cycles that can be interpreted directly from the eustatic reconstructions. We apply an interpretative algorithm to Paleogene sea level records and identify three orders of eustatic cycles longer than 1 Ma. However, the three-ordered cyclicity might not represent cycles of global eustatic change. First, only cycles of the highest of the established orders (with a timescale of 10 s of Ma) are coherent among different sea-level records. Second, the interpreted cycles are not supported by the compilation of the regional maximum flooding surfaces. Third, climatic history alone cannot explain the eustatic changes. Fourth, the interpreted cyclicity differs significantly from what is known about the tectonic control of eustasy. Fifth, there may be other orders higher than those established. The problem is rooted in (1) the fact that eustatic curves might not necessarily reflect global events (e.g., fluctuations shown on these curves may be artifacts related to regional tectonic activity) and (2) the possible weakness of Paleogene (especially Eocene) eustatic cyclicity and its significant “overprint” by regional tectonic activity. Our attempted analysis claims for significant improvement of the available eustatic reconstructions. Unfortunately, the regional stratigraphical data remain still insufficient to develop any alternative eustatic curve that can be further interpreted to understand the number of “actual” cycle orders.

• Sames, B.; Wagreich, M.; Wendler, J.E.; Haq, B.U.; Conrad, Clinton Phillips; Melinte-Dobrinescu, M.C.; Hu, X; Wendler, I; Wolfgring, E; Yilmaz, I.O. & Zorina, S.O. (2016). Review: Short-term sea-level changes in a greenhouse world — A view from the Cretaceous. Palaeogeography, Palaeoclimatology, Palaeoecology.  ISSN 0031-0182.  441, s 393- 411 . doi: 10.1016/j.palaeo.2015.10.045 Show summary

  This review provides a synopsis of ongoing research and our understanding of the fundamentals of sea-level change today and in the geologic record, especially as illustrated by conditions and processes during the Cretaceous greenhouse climate episode. We give an overview of the state of the art of our understanding on eustatic (global) versus relative (regional) sea level, as well as long-term versus short-term fluctuations and their drivers. In the context of the focus of UNESCO-IUGS/IGCP project 609 on Cretaceous eustatic, short-term sea-level and climate changes, we evaluate the possible evidence for glacio-eustasy versus alternative or additional mechanisms for continental water storage and release for the Cretaceous greenhouse and hothouse phases during which the presence of larger continental ice shields is considered unlikely. Increasing evidence in the literature suggests a correlation between long-period orbital cycles and depositional cycles that reflect sea-level fluctuations, implying a globally synchronized forcing of (eustatic) sea level. Fourth-order depositional sequences seem to be related to a ~ 405 ka periodicity, which most likely represents long-period orbital eccentricity control on sea level and depositional cycles. Third-order cyclicity, expressed as time-synchronous sea level falls of ~ 20 to 110 m on ~ 0.5 to 3.0 Ma timescales in the Cretaceous, are increasingly recognized as connected to climate cycles triggered by long-term astronomical cycles that have periodicity ranging from ~ 1.0 to 2.4 Ma. Future perspectives of research on greenhouse sea-level changes comprise a high-precision time-scale for sequence stratigraphy and eustatic sea-level changes and high-resolution marine to non-marine stratigraphic correlation.

• Veit, Emeline & Conrad, Clinton Phillips (2016). The impact of groundwater depletion on spatial variations in sea level change during the past century. Geophysical Research Letters.  ISSN 0094-8276.  43, s 3351- 3359 . doi: 10.1029/2012GL068118 Show summary

  Continental groundwater loss during the past century has elevated sea level by up to ~25 mm. The mass unloading associated with this depletion locally uplifts Earth's solid surface and depresses the geoid, leading to slower relative sea level rise near areas of significant groundwater loss. We computed spatial variations in sea level using a model of the solid Earth's response to estimates of groundwater depletion during the past century and find large negative deviations of ~0.4 mm/yr along the coastlines of western North America and southern Asia. This approximately corresponds to the difference between rates of sea level rise measured by tide gauges in these regions since 1930 and average rates inferred from global reconstructions. Groundwater-induced regional variations in sea level can be larger than those due to postglacial rebound and interseismic deformation and should become increasingly important in the future as both groundwater depletion and sea level rise accelerate.

• Ballmer, M. D.; Conrad, Clinton Phillips; Smith, E. I. & Johnsen, R. (2015). Intraplate volcanism at the edges of the Colorado Plateau sustained by a combination of triggered edge-driven convection and shear-driven upwelling. Geochemistry Geophysics Geosystems.  ISSN 1525-2027.  16, s 366- 379 . doi: 10.1002/2014GC005641 Show summary

  Although volcanism in the southwestern United States has been studied extensively, its origin remains controversial. Various mechanisms such as mantle plumes, upwelling in response to slab sinking, and small-scale convective processes have been proposed, but have not been evaluated within the context of rapidly shearing asthenosphere that is thought to underlie this region. Using geodynamic models that include this shear, we here explore spatiotemporal patterns of mantle melting and volcanism near the Colorado Plateau. We show that the presence of viscosity heterogeneity within an environment of asthenospheric shearing can give rise to decompression melting along the margins of the Colorado Plateau. Our models indicate that eastward shear flow can advect pockets of anomalously low viscosity toward the edges of thickened lithosphere beneath the plateau, where they can induce decompression melting in two ways. First, the arrival of the pockets critically changes the effective viscosity near the plateau to trigger small-scale edge-driven convection. Second, they can excite shear-driven upwelling (SDU), in which horizontal shear flow becomes redirected upward as it is focused within the low-viscosity pocket. We find that a combination of “triggered” edge-driven convection and SDU can explain volcanism along the margins of the Colorado Plateau, its encroachment toward the plateau's southwestern edge, and the association of volcanism with slow seismic anomalies in the asthenosphere. Geographic patterns of intraplate volcanism in regions of vigorous asthenospheric shearing may thus directly mirror viscosity heterogeneity of the sublithospheric mantle.

• Becker, T. W.; Schaeffer, A. J.; Lebedev, S. & Conrad, Clinton Phillips (2015). Toward a generalized plate motion reference frame. Geophysical Research Letters.  ISSN 0094-8276.  42, s 3188- 3196 . doi: 10.1002/2015GL063695 Show summary

  An absolute plate motion (APM) model is required to address issues such as the thermochemical evolution of Earth's mantle. All APM models have to rely on indirect inferences, including those based on hot spots and seismic anisotropy, each with their own set of uncertainties. Here, we explore a seafloor spreading-aligned reference frame. We show that this reference frame fits azimuthal seismic anisotropy in the uppermost mantle very well. The corresponding Euler pole is close to those of hot spot reference frames, ridge motion minimizing models, and geodynamic estimates of net rotation and predicts clear trench motion patterns. We conclude that a net rotation pole guided by the spreading-aligned model (at 64°E, 61°S, with moderate rotation of ∼ 0.2 … 0.3°/Myr) could indeed represent a standard, comprehensive reference frame for present-day plate motions with respect to the deep mantle.

• Conrad, Clinton Phillips (2015). How climate influences sea-floor topography. Science.  ISSN 0036-8075.  347, s 1204- 1205 . doi: 10.1126/science.aaa6813 Show summary

  Earth's sea floor is created along great volcanic ridges that spread at rates of a few centimeters per year beneath every ocean basin. The volcanic and tectonic processes at these mid-ocean ridges ultimately determine the characteristics of the sea floor and the oceanic crust beneath it. These processes occur beneath several kilometers of rock and seawater, seemingly far removed from climatic variations above sea level. Thus, it is surprising that both Crowley et al. (1), on page 1237 in this issue, and Tolstoy (2) found ice age periodicity in hilly topography on the flanks of ridges submerged beneath the Southern and Pacific Oceans.

• Becker, T. W.; Conrad, Clinton Phillips; Schaeffer, A. J. & Lebedev, S. (2014). Origin of azimuthal seismic anisotropy in oceanic plates and mantle. Earth and Planetary Science Letters.  ISSN 0012-821X.  401, s 236- 250 . doi: 10.1016/j.epsl.2014.06.014 Show summary

  Seismic anisotropy is ubiquitous in the Earth's mantle but strongest in its thermo-mechanical boundary layers. Azimuthal anisotropy in the oceanic lithosphere and asthenosphere can be imaged by surface waves and should be particularly straightforward to relate to well-understood plate kinematics and large-scale mantle flow. However, previous studies have come to mixed conclusions as to the depth extent of the applicability of paleo-spreading and mantle flow models of anisotropy, and no simple, globally valid, relationships exist. Here, we show that lattice preferred orientation (LPO) inferred from mantle flow computations produces a plausible global background model for asthenospheric anisotropy underneath oceanic lithosphere. The same is not true for absolute plate motion (APM) models. A ∼200 km thick layer where the flow model LPO matches observations from tomography lies just below the View the MathML source isotherm of a half-space cooling model, indicating strong temperature-dependence of the processes that control the development of azimuthal anisotropy. We infer that the depth extent of shear, and hence the thickness of a relatively strong oceanic lithosphere, can be mapped this way. These findings for the background model, and ocean-basin specific deviations from the half-space cooling pattern, are found in all of the three recent and independent tomographic models considered. Further exploration of deviations from the background model may be useful for general studies of oceanic plate formation and dynamics as well as regional-scale tectonic analyses.

• Ballmer, M.D.; Conrad, Clinton Phillips; Smith, E.I. & Harmon, N. (2013). Non-hotspot volcano chains produced by migration of shear-driven upwelling toward the East Pacific Rise. Geology.  ISSN 0091-7613.  41, s 479- 482 . doi: 10.1130/G33804.1 Show summary

  While most oceanic volcanism is associated with the passive rise of hot mantle beneath the spreading axes of mid-ocean ridges (MOR), volcanism occurring off-axis reflects intraplate upper-mantle dynamics and composition, yet is poorly understood. Off the south East Pacific Rise (SEPR), volcanism along the Pukapuka, Hotu-Matua, and Sojourn ridges has been attributed to various mechanisms, but none can reconcile its spatial, temporal, and geochemical characteristics. Our three-dimensional numerical models show that asthenospheric shear can excite upwelling and decompression melting at the tip of low-viscosity fingers that are propelled eastward by vigorous sublithospheric flow. This shear-driven upwelling is able to sustain intraplate volcanism that progresses toward the MOR, spreads laterally close to the axis, and weakly continues on the opposite plate. These predictions can explain the anomalously fast eastward progression of volcanism, and its spatial distribution near the SEPR. Moreover, for a heterogeneous mantle source involving a fertile component, the predicted systematics of volcanism can explain the geochemical trend along Pukapuka and the enriched anomaly of SEPR mid-oceanic ridge basalt at 16°–20.5°S. Our study highlights the role of horizontal asthenospheric flow and mantle heterogeneity in producing linear chains of intraplate volcanism independent of a (deep-rooted) buoyancy source.

• Conrad, Clinton Phillips (2013). The solid earth's influence on sea level. Geological Society of America Bulletin.  ISSN 0016-7606.  125, s 1027- 1052 . doi: 10.1130/B30764.1 Show summary

  Because it lies at the intersection of Earth’s solid, liquid, and gaseous components, sea level links the dynamics of the fluid part of the planet with those of the solid part of the planet. Here, I review the past quarter century of sea-level research and show that the solid components of Earth exert a controlling influence on the amplitudes and patterns of sea-level change across time scales ranging from years to billions of years. On the shortest time scales (100–102 yr), elastic deformation causes the ground surface to uplift instantaneously near deglaciating areas while the sea surface depresses due to diminished gravitational attraction. This produces spatial variations in rates of relative sea-level change (measured relative to the ground surface), with amplitudes of several millimeters per year. These sea-level “fingerprints” are characteristic of (and may help identify) the deglaciation source, and they can have significant societal importance because they will control rates of coastal inundation in the coming century. On time scales of 103–105 yr, the solid Earth’s time-dependent viscous response to deglaciation also produces spatially varying patterns of relative sea-level change, with centimeters-per-year amplitude, that depend on the time-history of deglaciation. These variations, on average, cause net seafloor subsidence and therefore global sea-level drop. On time scales of 106–108 yr, convection of Earth’s mantle also supports long-wavelength topographic relief that changes as continents migrate and mantle flow patterns evolve. This changing “dynamic topography” causes meters per millions of years of relative sea-level change, even along seemingly “stable” continental margins, which affects all stratigraphic records of Phanerozoic sea level. Nevertheless, several such records indicate sea-level drop of ∼230 m since a mid-Cretaceous highstand, when continental transgressions were occurring worldwide. This global drop results from several factors that combine to expand the “container” volume of the ocean basins. Most importantly, ridge volume decrease since the mid-Cretaceous, caused by an ∼50% slowdown in seafloor spreading rate documented by tectonic reconstructions, explains ∼250 m of sea-level fall. These tectonic changes have been accompanied by a decline in the volume of volcanic edifices on Pacific seafloor, continental convergence above the former Tethys Ocean, and the onset of glaciation, which dropped sea level by ∼40, ∼20, and ∼60 m, respectively. These drops were approximately offset by an increase in the volume of Atlantic sediments and net seafloor uplift by dynamic topography, which each elevated sea level by ∼60 m. Across supercontinental cycles, expected variations in ridge volume, dynamic topography, and continental compression together roughly explain observed sea-level variations throughout Pangean assembly and dispersal. On the longest time scales (109 yr), sea level may change as ocean water is exchanged with reservoirs stored by hydrous minerals within the mantle interior. Mantle cooling during the past few billion years may have accelerated drainage down subduction zones and decreased degassing at mid-ocean ridges, causing enough sea-level drop to impact the Phanerozoic sea-level budget. For all time scales, future advances in the study of sea-level change will result from improved observations of lateral variations in sea-level change, and a better understanding of the solid Earth deformations that cause them.

• Conrad, Clinton Phillips; Steinberger, B. & Torsvik, T.H. (2013). Conrad et al. reply. Nature.  ISSN 0028-0836.  503(7477), s E4- E4 . doi: 10.1038/nature1279 Show summary

  We thank Rudolph and Zhong for their Comment, which allows us to highlight important aspects of our original Letter. In particular, they have provided an example of plate motions at 300 million years (Myr) ago (see right column of figure 1 of ref. 1) in which the plate tectonic quadrupole is not representative of plate tectonic divergence patterns (that is, there is no divergence in the middle of their supercontinent, despite a divergent quadrupole there). However, our study does not claim that there should be a correspondence between quadrupole locations and the specific locations of plate tectonic divergence—instead we argue that plate tectonic dipole and quadrupole locations are representative of underlying mantle flow only.

• Conrad, Clinton Phillips; Steinberger, B. & Torsvik, T.H. (2013). Stability of active mantle upwelling revealed by net characteristics of plate tectonics. Nature.  ISSN 0028-0836.  498, s 479- 482 . doi: 10.1038/nature12203 Show summary

  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.

• Faccenna, C.; Becker, T.W.; Conrad, Clinton Phillips & Husson, L. (2013). Mountain building and mantle dynamics. Tectonics.  ISSN 0278-7407.  32, s 80- 93 . doi: 10.1029/2012TC003176 Show summary

  Mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large-scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen. A general, dynamic model for this process is still lacking. Here we propose that mountain belts can be classified between two end-members. First, those of “slab pull” type, where subduction is mainly confined to the upper mantle, and rollback trench motion lead to moderately thick crustal stacks, such as in the Mediterranean. Second, those of “slab suction” type, where whole-mantle convection cells (“conveyor belts”) lead to the more extreme expressions of orogeny, such as the largely thickened crust and high plateaus of present-day Tibet and the Altiplano. For the slab suction type, deep mantle convection produces the unique conditions to drag plates toward each other, irrespective of their nature and other boundary conditions. We support this hypothesis by analyzing the orogenic, volcanic, and convective history associated with the Tertiary formation of the Andes after ~40 Ma and Himalayas after collision at ~55 Ma. Based on mantle circulation modeling and tectonic reconstructions, we surmise that the forces necessary to sustain slab-suction mountain building in those orogens derive, after transient slab ponding, from the mantle drag induced upon slab penetration into the lower mantle, and from an associated surge of mantle upwelling beneath Africa. This process started at ~65–55 Ma for Tibet-Himalaya, when the Tethyan slab penetrated into the lower mantle, and ~10 Myr later in the Andes, when the Nazca slab did. This surge of mantle convection drags plates against each other, generating the necessary compressional forces to create and sustain these two orogenic belts. If our model is correct, the available geological records of orogeny can be used to decipher time-dependent mantle convection, with implications for the supercontinental cycle.

• Ruban, D. A. & Conrad, Clinton Phillips (2013). Late Silurian-Middle Devonian long-term shoreline shifts on the northern Gondwanan margin: Eustatic versus tectonic controls. Proceedings Geological Association.  ISSN 0016-7878.  124, s 883- 892 . doi: 10.1016/j.pgeola.2012.12.004 Show summary

  Long-term shoreline shifts reflect eustatic changes, tectonic activity, and sediment supply. Available lithostratigraphical data from northern Africa, Arabia, and the Tethys Hymalaya, coupled with facies interpretations, permit us to trace late Silurian–Middle Devonian long-term shoreline shifts across the northern Gondwanan margin and to compare them with constraints on global sea-level changes. Our analysis establishes a regression–transgression cycle. Its coincident global sea-level changes reveal the dominance of the eustatic control. A transgression–regression cycle observed in Arabia is best explained by regional subsidence. Our study highlights the importance of constraining the role of regional tectonics when interpreting shoreline shifts.

• van Summeren, J.; Gaidos, E. & Conrad, Clinton Phillips (2013). Magnetodynamo lifetimes for rocky, Earth-mass exoplanets with contrasting mantle convection regimes. Journal of Geophysical Research (JGR): Planets.  ISSN 2169-9097.  118, s 938- 951 . doi: 10.1002/jgre.20077 Show summary

  We used a thermal model of an iron core to calculate magnetodynamo evolution in Earth-mass rocky planets to determine the sensitivity of dynamo lifetime and intensity to planets with different mantle tectonic regimes, surface temperatures, and core properties. The heat flow at the core-mantle boundary (CMB) is derived from numerical models of mantle convection with a viscous/pseudoplastic rheology that captures the phenomenology of plate-like tectonics. Our thermal evolution models predict a long-lived (~8 Gyr) field for Earth and similar dynamo evolution for Earth-mass exoplanets with plate tectonics. Both elevated surface temperature and pressure-dependent mantle viscosity reduce the CMB heat flow but produce only slightly longer-lived dynamos (~8–9.5 Gyr). Single-plate (“stagnant lid”) planets with relatively low CMB heat flow produce long-lived (~10.5 Gyr) dynamos. These weaker dynamos can cease for several billions of years and subsequently reactivate due to the additional entropy production associated with inner core growth, a possible explanation for the absence of a magnetic field on present-day Venus. We also show that dynamo operation is sensitive to the initial temperature, size, and solidus of a planet's core. These dependencies would severely challenge any attempt to distinguish exoplanets with plate tectonics and stagnant lids based on the presence or absence of a magnetic field.

• Combes, M.; Grigné, Cecile; Husson, Laurent; Conrad, Clinton Phillips; Le Yaouanq, S.; Parenthoën, M.; Tisseau, C. & Tisseau, J. (2012). Multiagent simulation of evolutive plate tectonics applied to the thermal evolution of the Earth. Geochemistry Geophysics Geosystems.  ISSN 1525-2027.  13 . doi: 10.1029/2011GC004014 Show summary

  The feedback between plate tectonics and mantle convection controls the Earth's thermal evolution via the seafloor age distribution. We therefore designed the MACMA model to simulate time-dependent plate tectonics in a 2D cylindrical geometry with evolutive plate boundaries, based on multiagent systems that express thermal and mechanical interactions. We compute plate velocities using a local force balance and use explicit parameterizations to treat tectonic processes such as trench migration, subduction initiation, continental breakup and plate suturing. These implementations allow the model to update its geometry and thermal state at all times. Our approach has two goals: (1) to test how empirically- and analytically-determined rules for surface processes affect mantle and plate dynamics, and (2) to investigate how plate tectonics impact the thermal regime. Our predictions for driving forces, plate velocities and heat flux are in agreement with independent observations. Two time scales arise for the evolution of the heat flux: a linear long-term decrease and high-amplitude short-term fluctuations due to surface tectonics. We also obtain a plausible thermal history, with mantle temperature decreasing by less than 200 K over the last 3 Gyr. In addition, we show that on the long term, mantle viscosity is less thermally influential than tectonic processes such as continental breakup or subduction initiation, because Earth's cooling rate depends mainly on its ability to replace old insulating seafloor by young thin oceanic lithosphere. We infer that simple convective considerations alone cannot account for the nature of mantle heat loss and that tectonic processes dictate the thermal evolution of the Earth.

• Heuret, A.; Conrad, Clinton Phillips; Funiciello, F.; Lallemand, S. & Sandri, L. (2012). Relation between subduction megathrust earthquakes, trench sediment thickness and upper plate strain. Geophysical Research Letters.  ISSN 0094-8276.  39 . doi: 10.1029/2011GL050712 Show summary

  Giant earthquake (moment magnitude Mw ≥ 8.5) forecasts for subduction zones have been empirically related to both tectonic stresses and geometrical irregularities along the subduction interface. Both of these controls have been suggested as able to tune the ability of rupture to propagate laterally and, in turn, exert an important control on giant earthquake generation. Here we test these hypotheses, and their combined influence, by compiling a dataset of trench fill thickness (a proxy for smoothing of subducting plate relief by sediment input into the subduction channel) and upper plate strain (a proxy for the tectonic stresses applied to the subduction interface) for 44 segments of the global subduction network. We statistically compare relationships between upper plate strain, trench sediment thickness and maximal earthquake magnitude. We find that the combination of both large trench fill (≥1 km) and neutral upper plate strain explains spatial patterns of giant earthquake occurrence to a statistically significant degree. In fact, the concert of these two factors is more highly correlated with giant earthquake occurrence than either factor on its own. Less frequent giant earthquakes of lower magnitude are also possible at subduction zones with thinner trench fill and compressive upper plate strain. Extensional upper plate strain and trench fill < 0.5 km appear to be unfavorable conditions, as giant earthquakes have not been observed in these geodynamical environments during the last 111 years.

• Husson, L.; Conrad, Clinton Phillips & Faccenna, C. (2012). Plate motions, Andean orogeny, and volcanism above the South Atlantic convection cell. Earth and Planetary Science Letters.  ISSN 0012-821X.  317-318, s 126- 135 . doi: 10.1016/j.epsl.2011.11.040 Show summary

  The geometric and kinematic evolution of the Andes provides insight onto the nature of the force balance beneath the South American plate. While the Andean load is opposed on its western edge by the force induced by subduction of the Nazca plate, its more elusive eastern counterpart, which we explore herein, requires some contribution from the mantle beneath the South Atlantic. Using a mantle flow model, we show that the Andes owe their existence to basal drag beneath South America caused by a cylindrical convection cell under the South Atlantic. We find that the observed Andean uplift requires both westward push from active upwelling beneath Africa and westward drag toward the downgoing Nazca slab. These mutually-reinforcing downwellings and upwellings amount to 38% and 23% of the total driving force, respectively. Further decomposition reveals that the South Atlantic cell is most vigorous near its center, rendering the net drag force higher where the Andes also reach their highest elevation. Kinematic reconstructions suggest that the South Atlantic cell could have grown owing to the migration of the Nazca slab until ~ 50 Ma. We propose that from 50 Ma onwards, the cell may have ceased growing westward because (i) it had reached an optimal aspect ratio and (ii) the Nazca slab became anchored into the lower mantle. Continued westward motion of the plates, however, moved the surface expressions of spreading and convergence away from the upwelling and downwelling arms of this cell. Evidence for this scenario comes from the coeval tectonic, morphologic, and magmatic events in Africa and South America during the Tertiary.

• Husson, Laurent & Conrad, Clinton Phillips (2012). On the location of hotspots in the framework of mantle convection. Geophysical Research Letters.  ISSN 0094-8276.  39 . doi: 10.1029/2012GL052866 Show summary

  Putative mechanisms that have been proposed to explain intraplate “hotspot” volcanism extensively depart from the early plume theory, and many do not involve deep mantle flow. Here, we look for a relationship between hotspot volcanism and mantle flow using flow models excited by density anomalies inferred from seismic tomography. We show that previously identified major hotspots are preferentially located, to a high degree of statistical significance, above regions of positive divergence of horizontal shear tractions beneath the lithosphere. This observation renders it difficult to discard some contribution of mantle flow as a control on hotspot volcanism and instead suggests that mantle plumes are drawn toward, and conveyed by, mantle upwellings (either active or passive), which are revealed by the positive stress divergence. This allows us to exclude a variety of external or shallow mechanisms for the major hotspots. Because we also find that many secondary hotspots do fall at random locations with respect to mantle flow, we emphasize that alternative processes are also required to trigger the less productive volcanism.

• Natarov, Svetlana & Conrad, Clinton Phillips (2012). The role of Poiseuille flow in creating depth-variation of asthenospheric shear. Geophysical Journal International.  ISSN 0956-540X.  190, s 1297- 1301 . doi: 10.1111/j.1365-246X.2012.05562.x Show summary

  Asthenospheric flow accommodates differential shear between plate and mantle motions (Couette flow) and hosts additional flow driven by horizontal pressure gradients (Poiseuille flow) that may be associated with mantle upwelling and subduction. Large uncertainties in the upper mantle flow field and its rheological structure have thus far hindered our ability to constrain the relative importance of Couette and Poiseuille flows in the asthenosphere. However, quantifying the relative contributions of asthenospheric Couette and Poiseuille flows and determining the pattern of their distribution around the globe could help discriminate among competing theories of asthenospheric origin and shed light on thermal history of the Earth. We propose a new method to quantify asthenospheric Poiseuille flow using observations of the depth-dependence of azimuthal seismic anisotropy, which can be obtained from frequency-dependent surface wave tomography models. In particular, we employ a simple 1-D Couette-Poiseuille flow model and analytically solve for depth-profiles of the strain axis orientations, which approximates the orientations of azimuthal seismic anisotropy. We show that Couette-Poiseuille flow induces rotation of azimuthal seismic anisotropy with depth provided that the horizontal pressure gradient has a component transverse to plate motion. We then construct an algorithm that uses depth rotations of azimuthal anisotropy to invert for horizontal pressure gradients everywhere in the asthenosphere and test it on a global numerical mantle flow model. A comparison of pressure gradients predicted using our method with those computed directly from the numerical model shows that our algorithm is stable and accurate, unless the pressure gradient is nearly parallel to plate motion. Applying this method to seismic data will require additional constraints on asthenospheric geometry and viscosity structure. In the numerical model, we establish that Poiseuille flow drives ∼40 per cent of the total flow velocity amplitude in the asthenosphere, which indicates that pressure gradients from mantle convection may be an important component of asthenospheric dynamics that can, in principle, be constrained seismically.

• Ruban, D.A.; Zorina, S.O.; Conrad, Clinton Phillips & Afanasieva, N.I. (2012). In quest of Paleocene global-scale transgressions and regressions: constraints from a synthesis of regional trends. Proceedings Geological Association.  ISSN 0016-7878.  123, s 7- 18 . doi: 10.1016/j.pgeola.2011.08.003 Show summary

  Chronostratigraphically-justified records of regional transgressions and regressions are important for understanding the nature of the Paleocene shoreline shifts on a global scale. Review of previously synthesized data from 7 tectonically “stable” regions, namely the eastern Russian Platform, Northwestern Europe, Northwestern Africa, Northeastern Africa, the Arabian Platform, the northern Gulf of Mexico, and Southern Australia, allows a comparison of transgressions and regressions interpreted in these regions. No common patterns are found in the early Danian and late Selandian, which reflects small or zero eustatic fluctuations that are overwhelmed locally on coastlines by regional tectonic motions and local changes in dynamic support of surface topography by mantle flow. Sea level was stabilized during these stages by a warm climate and a lack of planetary-scale tectonic changes. We have detected a middle–late Danian regression that occurred in 5 of 7 study regions, and can be explained by glacial advance at ∼62–63 Ma or by concurrent subduction of the Izanagi–Pacific ridge beneath eastern Asia. An early–middle Selandian transgression also occurred in 5 regions, probably, as a result of a hyperthermal at ∼61 Ma that coincided with emplacement of large igneous provinces in the oceanic domain. Both events are characterized by significant diachroneity, which can also be explained by the influence of regional tectonic subsidence or uplift. Results of the present study permit us to propose a tentative framework for a new Paleocene eustatic curve that is constrained globally using available records of transgressions and regressions.

• van Summeren, Joost; Conrad, Clinton Phillips & Lithgow-Bertelloni, C. (2012). The importance of slab pull and a global asthenosphere to plate motions. Geochemistry Geophysics Geosystems.  ISSN 1525-2027.  13 . doi: 10.1029/2011GC003873 Show summary

  Earth's subducting plates move 3–4 times faster than its overriding plates, but it remains unclear whether these contrasting plate speeds are caused by additional pull from subducting slabs or by increased viscous drag on the lithosphere-asthenosphere boundary beneath deeply-protruding continental roots. To investigate the relative importance of plausible controls, we predicted global patterns of plate motions using numerical models that incorporate the influence of subducting slabs, convective mantle flow, and continental roots. From the mantle convection models, we computed a set of dynamically consistent plate velocities by balancing forces that drive and resist the motion of each plate. When deep continental roots anchor to the sub-asthenospheric upper mantle, the calculated patterns of plate motions are close to the observations if only ∼20% of (excess) upper mantle slab weight contributes to the slab pull force. However, this small contribution causes plates to move too slowly on average unless mantle viscosity is a factor of ∼2 lower than expected from post-glacial rebound. In contrast, we show that predicted and observed plate motions are more easily reconciled if even the deepest continental roots are underlain by a low-viscosity layer and at least half of (excess) upper mantle slab weight contributes to the slab pull force. This preferred scenario agrees with recent seismological evidence for a global asthenosphere and previously proposed mechanisms for partial decoupling of slabs from surface plates.

• Bianco, Todd A.; Conrad, Clinton Phillips & Smith, Eugene I. (2011). Time-dependence of intraplate volcanism caused by shear-driven upwelling of low-viscosity regions within the asthenosphere. Journal of Geophysical Research (JGR): Solid Earth.  ISSN 2169-9313.  116 . doi: doi:10.1029/2011JB008270 Show summary

  Although volcanism far from tectonic boundaries is likely due to upwelling near the lithospheric base, the convective processes that induce upwelling are unclear. Numerical models show that asthenospheric shear can be deflected upward by lateral viscosity variations within the asthenosphere, producing “shear‐driven upwelling” (SDU). To constrain the rate, duration, and surface expression of intraplate volcanism caused by SDU, we simulate 2‐D flow and peridotite melting in the upper 200 km of the mantle. Asthenospheric shear is driven by lithospheric plates with different thicknesses moving at 3–9 cm/yr, and the initial low‐viscosity region is a rectangularly shaped pocket with an imposed viscosity that is 2 orders of magnitude smaller than the surrounding asthenosphere. Melting decreases as the pocket deforms and reaches steady state after 3–12 Myr. The age progression of surface volcanism is nearly stationary in the reference frame of the plate, which distinguishes SDU from hot spot volcanism. Similar behavior occurs if the viscosity heterogeneity is induced by variations in the water content of mantle peridotite. If the pocket's low viscosity is caused by excess temperature, buoyant upwelling of the entire pocket dominates volcanism. Differences in the time dependence of volcanism associated with damp and warm pockets may help identify which type of mantle heterogeneity and associated dynamic process best explains weak, intermittent, intraplate volcanism with no obvious age progression. We suggest that asthenospheric shear induced by plate motions and global mantle flow, by exciting SDU, drives some of the non–hot spot small‐scale volcanism that occurs away from plate boundaries.

• Conrad, Clinton Phillips; Bianco, Todd A.; Smith, Eugene I. & Wessel, Paul (2011). Patterns of intraplate volcanism controlled by asthenospheric shear. Nature Geoscience.  ISSN 1752-0894.  4, s 317- 321 . doi: doi:10.1038/ngeo1111 Show summary

  Most of Earth’s volcanism occurs at plate boundaries, in association with subduction or rifting. A few high-volume volcanic fields are observed both at plate boundaries and within plates, fed by plumes upwelling from the deep mantle. The remaining volcanism is observed away from plate boundaries. It is typically basaltic, effusive and low volume, occurring within continental interiors or creating seamounts on the ocean floor. This intraplate volcanism has been attributed to various localized processes such as cracking of the lithosphere, small-scale convection in the mantle beneath the lithosphere or shear-induced melting of low-viscosity pockets of asthenospheric mantle that have become embedded along the base of the lithosphere. Here we compare the locations of observed intraplate volcanism with global patterns of mantle flow from a numerical model. We find a correlation between recent continental and oceanic intraplate volcanism and areas of the asthenosphere that are experiencing rapid shear due to mantle convection. We detect particularly high correlations in the interior of the continents of western North America, eastern Australia, southern Europe and Antarctica, as well as west of the East Pacific Rise in the Pacific Ocean. We conclude that intraplate volcanism associated with mantle convection is best explained by melting caused by shear flow within the asthenosphere, whereas other localized processes are less important.

• van Summeren, Joost; Conrad, Clinton Phillips & Gaidos, Eric (2011). Mantle convection, plate tectonics, and volcanism on hot exo-Earths. Astrophysical Journal Letters.  ISSN 2041-8205.  736 . doi: 10.1088/2041-8205/736/1/L15 Show summary

  Recently discovered exoplanets on close-in orbits should have surface temperatures of hundreds to thousands of Kelvin. They are likely tidally locked and synchronously rotating around their parent stars and, if an atmosphere is absent, have surface temperature contrasts of many hundreds to thousands of Kelvin between permanent day and night sides. We investigated the effect of elevated surface temperature and strong surface temperature contrasts for Earth-mass planets on the (1) pattern of mantle convection, (2) tectonic regime, and (3) rate and distribution of partial melting, using numerical simulations of mantle convection with a composite viscous/pseudo-plastic rheology. Our simulations indicate that if a close-in rocky exoplanet lacks an atmosphere to redistribute heat, a gsim400 K surface temperature contrast can maintain an asymmetric degree 1 pattern of mantle convection in which the surface of the planet moves preferentially toward subduction zones on the cold night side. The planetary surface features a hemispheric dichotomy, with plate-like tectonics on the night side and a continuously evolving mobile lid on the day side with diffuse surface deformation and vigorous volcanism. If volcanic outgassing establishes an atmosphere and redistributes heat, plate tectonics is globally replaced by diffuse surface deformation and volcanism accelerates and becomes distributed more uniformly across the planetary surface.

View all works in Cristin

• Kiraly, Agnes; Conrad, Clinton Phillips; Domeier, Mathew & Hansen, Lars (2019). Does anisotropic mantle viscosity impede changes in plate motion?.
• Conrad, Clinton Phillips & Domeier, Mathew (2018). Tracing the edges of the LLSVPs in the spatial distribution of seamount volcanism.
• Crameri, Fabio; Conrad, Clinton Phillips; Montesi, L. & Lithgow-Bertelloni, C. (2018). “`Ocean-Plate Tectonics’: The importance of the mantle framework.
• Domeier, Mathew; Conrad, Clinton Phillips & Selway, K. (2018). A link between seamount volcanism and structures of the deep Earth.
• Ghosh, A.; Paul, J. & Conrad, Clinton Phillips (2018). The relation between tractions and strain rate at the base of the lithosphere: Key to understanding cratonic stability.
• Hartman, Robert; Ebbing, Jörg & Conrad, Clinton Phillips (2018). Influence of upper mantle viscosity variations on sea level change and GIA - A case study for Antarctic deglaciation models.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2018). Stabilization of thermochemical piles by compositional viscosity contrasts.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2018). Stabilization of thermochemical piles by compositional viscosity contrasts. EGU Gen. Assembly, EGU2018-12950.
• Karlsen, Krister Stræte; Conrad, Clinton Phillips & Magni, Valentina (2018). Deep water cycling and sea level changes on a supercontinental time scale.
• Karlsen, Krister Stræte; Conrad, Clinton Phillips & Magni, Valentina (2018). Deep water recycling and cyclic sea level change on a supercontinental time scale.
• Karlsen, Krister Stræte; Conrad, Clinton Phillips & Magni, Valentina (2018). Deep water recycling and cyclic sea level change on a supercontinental time scale.
• Kiraly, Agnes; Conrad, Clinton Phillips & Hansen, L. (2018). Geodynamic consequences of anisotropic mantle viscosity.
• Kiraly, Agnes; Conrad, Clinton Phillips & Hansen, L. (2018). Geodynamic consequences of anisotropic mantle viscosity.
• Uppalapati, Sruthi; Rolf, Tobias; Crameri, Fabio; Conrad, Clinton Phillips & Werner, Stephanie C. (2018). How Venus’ young surface came to be: New insights from 2D and 3D modelling.
• Uppalapati, Sruthi; Rolf, Tobias; Crameri, Fabio; Conrad, Clinton Phillips & Werner, Stephanie C. (2018). How Venus’ young surface came to be: New insights from 2D and 3D modelling.
• Wessel, Paul & Conrad, Clinton Phillips (2018). Absolute plate and plume motions and implications for true polar wander.
• Boudinier, G.; Wessel, P. & Conrad, Clinton Phillips (2017). Plume-spotting: Deriving the absolute motion of hotspots and plates.
• Conrad, Clinton Phillips (2017). Addressing Outstanding Problems in Deep Earth Dynamics Using Data and Models.
• Conrad, Clinton Phillips (2017). The deep carbon cycle and mantle-lithosphere dynamics: Looking backward from today.
• Conrad, Clinton Phillips & Domeier, M. (2017). Tracing the edges of the LLSVPs in the spatial distribution of seamount volcanism.
• Conrad, Clinton Phillips & Hansen, L. (2017). Interplay between Tectonic Plate Motions, Anisotropic Viscosity, and the Development of Rock Fabrics in the Asthenosphere.
• Conrad, Clinton Phillips; Selway, K.; Hirschmann, M.M.; Ballmer, M.D. & Wessel, P. (2017). Constraints on volumes and patterns of asthenospheric melt from the space-time distribution of seamounts.
• Conrad, Clinton Phillips; Selway, K.; Hirschmann, M.M.; Ballmer, M.D. & Wessel, P. (2017). Volumes and patterns of asthenospheric melt inferred from the space-time distribution of seamounts.
• Conrad, Clinton Phillips; Steinberger, B. & Torsvik, T.H. (2017). Sea Level Change due to Time-Dependent Long-Wavelength Dynamic Topography Inferred from Plate Tectonic Reconstructions.
• Conrad, Clinton Phillips; Steinberger, B. & Torsvik, T.H. (2017). Tectonic reconstructions of dynamic topography and sea level.
• Conrad, Clinton Phillips; Watkins, C.E.; Steinberger, B. & Torsvik, T.H. (2017). Misshapen Earth: Inferring dynamic topography from bathymetry and plate motions.
• Dangendorf, S.; Marcos, M.; Wöppelmann, G.; Conrad, Clinton Phillips; Frederikse, T. & Riva, R. (2017). A reconciled estimate of 20th century global mean sea-level rise.
• Heyn, B.; Conrad, Clinton Phillips & Trønnes, R. (2017). Stabilizing effect of a chemical viscosity contrast on LLSVP structures.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2017). Stabilizing effect of a chemical viscosity contrast on LLSVP structures Abstr., Gordon Res. Conf. on Chemical and dynamical evolution of Earth's deep interior, from formation to today. Mount Holyoke College, South Hadley, MA.
• Steinberger, B & Conrad, Clinton Phillips (2017). Are Superplumes a Myth?.
• Togia, H.F.R.; Conrad, Clinton Phillips; Wessel, P. & Ito, G. (2017). New constraints on temporal variations in Hawaiian plume buoyancy flux.
• Wessel, P. & Conrad, Clinton Phillips (2017). Assessing Pacific absolute plate and plume motions.
• Boudinier, G.P.; Wessel, Paul & Conrad, Clinton Phillips (2016). Modeling absolute plate and plume motions.
• Conrad, Clinton Phillips (2016). Earth Modeling.
• Conrad, Clinton Phillips (2016). Geodynamics of Earth's Lithosphere and Mantle.
• Conrad, Clinton Phillips (2016). Inferring Earth’s Long-Wavelength Dynamic Topography from Bathymetry and Plate Motions.
• Conrad, Clinton Phillips (2016). Water Planet.
• Conrad, Clinton Phillips; Ballmer, M.D.; Harmon, N. & Smith, E.I. (2016). The shear-driven upwelling: Linking intraplate volcanism to global mantle flow.
• Conrad, Clinton Phillips; Steinberger, Bernhard & Torsvik, T.H. (2016). Inferring the time history of long-wavlength dynamic topography using tectonic reconstructions and global mantle flow models.
• Watkins, Clifford Evan & Conrad, Clinton Phillips (2016). Constraints on dynamic topography from asymmetric subsidence across seafloor unperturbed by volcanism surrounding the Mid-Atlantic Ridge and East Pacific Rise.

View all works in Cristin