Adrian Lenardic

Longevity and stability of cratonic lithosphere: Insights from numerical simulations of coupled mantle convection and continental tectonics

[1] The physical conditions required to provide for the tectonic stability of cratonic crust and for the relative longevity of deep cratonic lithosphere within a dynamic, convecting mantle are explored through a suite of numerical simulations. The simulations allow chemically distinct continents to reside within the upper thermal boundary layer of a thermally convecting mantle layer. A rheologic formulation, which models both brittle and ductile behavior, is incorporated to allow for plate-like behavior and the associated subduction of oceanic lithosphere. Several mechanisms that may stabilize cratons are considered. The two most often invoked mechanisms, chemical buoyancy and/or high viscosity of cratonic root material, are found to be relatively ineffective if cratons come into contact with subduction zones. High root viscosity can provide for stability and longevity but only within a thick root limit in which the thickness of chemically distinct, high-viscosity cratonic lithosphere exceeds the thickness of old oceanic lithosphere by at least a factor of 2. This end-member implies a very thick mechanical lithosphere for cratons. A high brittle yield stress for cratonic lithosphere as a whole, relative to oceanic lithosphere, is found to be an effective and robust means for providing stability and lithospheric longevity. This mode does not require exceedingly deep strength within cratons. A high yield stress for only the crustal or mantle component of the cratonic lithosphere is found to be less effective as detachment zones can then form at the crust-mantle interface which decreases the longevity potential of cratonic roots. The degree of yield stress variations between cratonic and oceanic lithosphere required for stability and longevity can be decreased if cratons are bordered by continental lithosphere that has a relatively low yield stress, i.e., mobile belts. Simulations that combine all the mechanisms can lead to crustal stability and deep root longevity for model cratons over several mantle overturn times, but the dominant stabilizing factor remains a relatively high brittle yield stress for cratonic lithosphere.

Some thoughts on the stability of cratonic lithosphere: Effects of buoyancy and viscosity

Continental cratons have not experienced major tectonic disruptions over a timescale of 109 years. The thickness of cratonic lithosphere also appears to have changed little over this timescale. These observations are often attributed to the presence of chemically buoyant and/or highly viscous subcratonic roots. Simple physical scaling relationships are developed to explore the buoyancy and/or viscosity conditions required to stabilize such roots against large-scale deformation and rapid remixing into the mantle. The scalings are tested using idealized numerical simulations with good general agreement. Applied to Earth, the scalings suggest that (1) buoyancy alone is unlikely to stabilize cratonic roots and (2) if root viscosity is to provide stability into the Archean, then roots must be 103 times as viscous as the mantle. Based on available experimental data, root dehydration cannot account for the required viscosity increase. Temperature-dependent viscosity can stabilize roots, but it does so at the expense of stagnating the entire mantle lithosphere, i.e., at the expense of sacrificing plate tectonics. This suggests that the plastic yielding properties of rocks at low temperatures will need to be more directly accounted for in future experiments exploring root stability.

The role of mobile belts for the longevity of deep cratonic lithosphere: The Crumple Zone Model

Many Archean cratons are surrounded by Proterozoic mobile belts that have experienced episodes of tectonic re-activation over their lifetimes. This suggests that mobile belt lithosphere may be associated with long lived, inherited weakness. It is proposed that the proximity of this weakness can increase the longevity of deep Archean lithosphere by buffering Archean cratons from mantle derived stresses. The physical plausibility of this idea is explored through numerical simulations of mantle convection that include continents and allow for material rheologies that model the combined brittle and ductile behavior of the lithosphere. Within the simulations, the longevity of deep cratonic lithosphere does increase if it is buffered by mobile belts that can fail at relatively low stress levels.

Continental arc–island arc fluctuations, growth of crustal carbonates, and long-term climate change

The Cretaceous to early Paleogene (ca. 140–50 Ma) was characterized by a greenhouse baseline climate, driven by elevated concentrations of atmospheric CO_2. Hypotheses for the elevated CO_2 concentrations invoke an increase in volcanic CO_2 production due to higher oceanic crust production rates, higher frequency of large igneous provinces, or increases in pelagic carbonate deposition, the last leading to enhanced carbonate subduction into the mantle source regions of arc volcanoes. However, these are not the only volcanic sources of CO_2 during this time interval. We show here that ocean-continent subduction zones, manifested as a global chain of continental arc volcanoes, were as much as 200% longer in the Cretaceous and early Paleogene than in the late Paleogene to present, when a cooler climate prevailed. In particular, many of these continental arcs, unlike island arcs, intersected ancient continental platform carbonates stored on the continental upper plate. We show that the greater length of Cretaceous–Paleogene continental arcs, specifically carbonate-intersecting arcs, could have increased global production of CO_2 by at least 3.7–5.5 times that of the present day. This magmatically driven crustal decarbonation flux of CO_2 through continental arcs exceeds that delivered by Cretaceous oceanic crust production, and was sufficient to drive Cretaceous–Paleogene greenhouse conditions. Thus, carbonate-intersecting continental arc volcanoes likely played an important role in driving greenhouse conditions in the Cretaceous–Paleogene. If so, the waning of North American and Eurasian continental arcs in the Late Cretaceous to early Paleogene, followed by a fundamental shift in western Pacific subduction zones ca. 52 Ma to an island arc–dominated regime, would have been manifested as a decline in global volcanic CO_2 production, prompting a return to an icehouse baseline in the Neogene. Our analysis leads us to speculate that long-term (>50 m.y.) greenhouse-icehouse oscillations may be linked to fluctuations between continental- and island arc–dominated states. These tectonic fluctuations may result from large-scale changes in the nature of subduction zones, changes we speculate may be tied to the assembly and dispersal of continents. Specifically, dispersal of continents may drive the leading edge of continents to override subduction zones, resulting in continental arc volcanism, whereas assembly of continents or closing of large ocean basins may be manifested as large-scale slab rollback, resulting in the development of intraoceanic volcanic arcs. We suggest that greenhouse-icehouse oscillations are a natural consequence of plate tectonics operating in the presence of continental masses, serving as a large capacitor of carbonates that can be episodically purged during global flare-ups in continental arcs. Importantly, if the global crustal carbonate reservoir has grown with time, as might be expected because platform carbonates on continents do not generally subduct, the greenhouse-driving potential of continental arcs would have been small during the Archean, but would have increased in the Neoproterozoic and Phanerozoic after a significant reservoir of crustal carbonates had formed in response to the evolution of life and the growth of continents.

Growth of the hemispheric dichotomy and the cessation of plate tectonics on Mars

[1] Although Mars is currently not tectonically active, it may have experienced plate tectonics early in its history. The southern hemisphere of Mars possesses a thick crust which probably renders the lithosphere positively buoyant. In this paper we present numerical and scaling arguments which show that if the area of positively buoyant lithosphere grows beyond a critical fraction (� 50% for Mars), plate tectonics will stop. Heat transfer through the buoyant lithosphere is inefficient, which causes mean mantle temperatures to increase as the surface area of buoyant lithosphere increases. The resulting reduction in mantle viscosity reduces shear stresses; if these shear stresses drop below the yield strength of the lithosphere, plate motions will cease and the planet will behave as a one-plate system. Thus the end of plate tectonics on Mars is a natural consequence of the growth of the southern highlands. Similar arguments for the Earth predict that it should operate in the plate tectonic regime now but that it may have experienced stagnant lid convection in the past. INDEX TERMS: 5475 Planetology: Solid Surface Planets: Tectonics (8149); 5430 Planetology: Solid Surface Planets: Interiors (8147); 5418 Planetology: Solid Surface Planets: Heat flow; 5455 Planetology: Solid Surface Planets: Origin and evolution; KEYWORDS: hemispheric dichotomy, mantle convection, thermal evolution, rheology

Continents, supercontinents, mantle thermal mixing, and mantle thermal isolation: Theory, numerical simulations, and laboratory experiments

Super-continental insulation refers to an increase in mantle temperature below a supercontinent due to the heat transfer inefficiency of thick, stagnant continental lithosphere relative to thinner, subducting oceanic lithosphere. We use thermal network theory, numerical simulations, and laboratory experiments to provide tighter physical insight into this process. We isolate two end-member dynamic regimes. In the thermally well mixed regime the insulating effect of continental lithosphere can not cause a localized increase in mantle temperature due to the efficiency of lateral mixing in the mantle. In this regime the potential temperature of the entire mantle is higher than it would be without continents, the magnitude depending on the relative thickness of continental and oceanic lithosphere (i.e., the insulating effects of continental lithosphere are communicated to the entire mantle). Thermal mixing can be short circuited if subduction zones surround a supercontinent or if the convective flow pattern of the mantle becomes spatially fixed relative to a stationary supercontinent. This causes a transition to the thermal isolation regime: The potential temperature increases below a supercontinent whereas the potential temperature below oceanic domains drops such that the average temperature of the whole mantle remains constant. Transition into this regime would thus involve an increase in the suboceanic viscosity, due to local cooling, and consequently a decrease in the rate of oceanic lithosphere overturn. Transition out of this regime can involve the unleashing of flow driven by a large lateral temperature gradient, which will enhance global convective motions. Our analysis highlights that transitions between the two states, in either direction, will effect not only the mantle below a supercontinent but also the mantle below oceanic regions. This provides a larger set of predictions that can be compared to the geologic record to help determine if a hypothesized super-continental thermal effect did or did not occur on our planet.

Viscous coupling at the lithosphere-asthenosphere boundary

Tectonic plate motions reflect dynamical contributions from subduction processes (i.e., classical “slab-pull” forces) and lateral pressure gradients within the asthenosphere (“asthenosphere-drive” forces), which are distinct from gravity forces exerted by elevated mid-ocean ridges (i.e., classical “ridge-push” forces). Here we use scaling analysis to show that the extent to which asthenosphere-drive contributes to plate motions depends on the lateral dimension of plates and on the relative viscosities and thicknesses of the lithosphere and asthenosphere. Whereas slab-pull forces always govern the motions of plates with a lateral extent greater than the mantle depth, asthenosphere-drive forces can be relatively more important for smaller (shorter wavelength) plates, large relative asthenosphere viscosities or large asthenosphere thicknesses. Published plate velocities, tomographic images and age-binned mean shear wave velocity anomaly data allow us to estimate the relative contributions of slab-pull and asthenosphere-drive forces for the motions of the Atlantic and Pacific plates. Whereas the Pacific plate is driven largely by slab pull, the Atlantic plate is predicted to be strongly driven by basal forces related to viscous coupling to strong asthenospheric flow, consistent with recent observations related to the stress state of North America. In addition, compared to the East Pacific Rise (EPR), the relatively large lateral pressure gradient near the Mid-Atlantic Ridge (MAR) is expected to produce significantly steeper dynamic topography. Thus, the relative importance of this plate-driving force may partly explain why the flanking topography at the EPR is smoother than at the MAR. Our analysis also indicates that this plate-driving force was more significant, and heat loss less efficient, in Earths hotter past compared with its cooler present state. This type of trend is consistent with thermal history modeling results which require less efficient heat transfer in Earths past.

Creation and Preservation of Cratonic Lithosphere: Seismic Constraints And Geodynamic Models

Cratons are areas of continental lithosphere that exhibit long-term stability against deformation. Seismic evidence suggests that cratonic lithosphere may have formed via thrust stacking of proto-cratonic lithosphere. We conducted numerical simulations and scaling analysis to test this hypothesis, as well as to elucidate mechanisms for stabilization. We found that formation of cratonic lithosphere via thrust stacking is most viable for buoyant and viscous lithosphere that is thin and/or possesses low effective friction coefficients. These conditions lead to low integrated yield strength within proto-cratonic lithosphere that allows it to fail in response to convection-generated stresses. Specifically, formation via thrust stacking is viable for lithosphere with chemical to thermal buoyancy ratios of B = 0.75-1.5, viscosity contrasts between the lithosphere and convective mantle of AT) > 10 2 , and friction coefficients of μ = 0.05-0.1. Preservation depends on the balance between the chemical lithospheres integrated yield and convection-generated stresses. The physical process of thrust stacking generates a thickened cratonic root. This provides a higher integrated yield stress within cratons, which is more conducive to stability subsequent to formation. Increased friction coefficient values, due to dehydration, can also provide higher integrated yield stresses within cratons. To provide long-term stability, integrated yield stresses must be great enough to offset future mantle convection-generated stresses, which can increase with time as the mantle viscosity increases due to cooling. Thin or rehydrated cratonic lithosphere may not provide stability against the increasing convective stresses, thus providing an explanation as to why some cratons are not long-lived.

Thermal convection below a conducting lid of variable extent: Heat flow scalings and two-dimensional, infinite Prandtl number numerical simulations

Theoretical heat flow scalings are presented for free thermal convection occurring below a conducting lid. Cases in which the lid covers the full extent of the convecting fluid and in which the lid has a variable lateral extent are treated. The scaling predictions are tested against a suite of two-dimensional numerical simulations that solve the full mathematical equations describing infinite Prandtl number thermal convection occurring below a conducting lid. The scaling predictions and simulation results show reasonably good agreement for Rayleigh numbers greater than 10 7 . The scaling predictions are also tested against previous numerical simulations of finite Prandtl number thermal convection below a laterally uniform lid. Scaling predictions and simulations results again show reasonable agreement.

Earth's punctuated tectonic evolution: cause and effect

Abstract Peaks in the Precambrian preserved crustal record are associated with major volcanic, tectonic and climatic events. These include addition of juvenile continental crust, voluminous high-temperature volcanism, massive mantle depletion, widespread orogeny and mineralization, large apparent polar wander velocity spikes, and subsequent palaeomagnetic intensity increases. These events impinge on the glaciation record, atmospheric and ocean chemistry, and on the rise of oxygen. Here we summarize and assess a number of geodynamic models that have been proposed to explain the observed episodicity in the Precambrian record. We find that episodic behaviour from nonlinear slab-driven models best explains the observed record. Examples of such slab-driven systems include mantle avalanches or episodic subduction. In these cases, rapid descent of slabs into the mantle drives fast plate motions and convergence at the surface. This is accompanied by large-scale upwellings of deep hot mantle, which contribute to voluminous volcanism. Further modelling will determine the relative importance of each mechanism, and reinforce the fundamental contribution of the mantle to the evolution of Earths surface systems.