1. SEISMOLOGY GELOGY GEOCHEMISTRY 7 Myr 29 Myr WHAT IS A CRATON WHAT ARE THE KEY PHYSICAL FACTORS THAT MAKE A CRATON WHAT IT IS 20 Myr 50 Myr 100 Myr 23 Myr ROOT BUOYANCY ROOT VISCOSITY MOBILE BELTS Continent/Mantle Yield Ratio = 2 root only root + crust Continent/Mantle Yield Ratio = 3 Continent/Mantle Yield Ratio = 4 root only root + crust root only root + crust 50 Myr 100 Myr 50 Myr MIXED FACTORS YIELD PROPERTIES 15 Myr MODEL SET-UP: SUMMARY 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 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. A rheologic formulation, that 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 mechanism, chemical buoyancy and/or high viscosity of cratonic root material, are found to be relatively ineffective if cratons come into contact with subduction zones. A high brittle yield stress for cratonic lithosphere as a whole, relative to oceanic lithosphere, is found to be most effective. 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. A Word or Two on Model Set Up : The model set up is shown at the far right. The simulations send a continent with a cratonic root into an incipient subduction zone. The idea is that if key physical factors can stabilize a craton in this setting then they can do so anywhere. It is possible that cratons avoid subduction zones but if this is the case then the physical properties of cratonic lithosphere are not the keys to root longevity and crustal stability as is generally assumed. We do acknowledge this possibility and explore it in the “mobile belt models” section. A Buoyant Word or Two: The chemical buoyancy of cratonic roots is the most often invoked potential means of providing stability & longevity to cratonic lithosphere. Although buoyancy can prevent cratonic lithosphere from sinking into the mantle under its own weight, via a Rayleigh-Taylor type instability, it can not provide for tectonic stability if a craton comes into contact with a subducting slab. Subduction forces can cause deformation of cratonic crust and roots regardless of root buoyancy. This deformation can cause detachments to form at the crust-root interface which means crustal buoyancy can not add to the longevity potential of root material. Deformation can also cause root material to be ‘stretched’ into smaller pieces. This lowers the integrated buoyancy of a root section allowing the negative thermal buoyancy of a slab to overwhelm chemical buoyancy leading to the recycling of root material. A Viscous Word or Two: A high viscosity of root material, due to cool temperature and/or dryness, is another often invoked potential means of providing for cratonic stability and root longevity. Although appealing it is worth remembering that oceanic lithosphere has a high viscosity itself yet it is hardly long lived. This hits at a key point: the strength of the lithosphere does not depend solely on its ductile behavior but also on its brittle behavior. The simulation results to the right show that root material with a high ductile viscosity relative to bulk mantle can still be recycled through brittle failure. High stresses can build in a high viscosity root and if these stresses exceed the yield strength of the root material then it will fail in a brittle mode. This allows the root to be “broken” into smaller sections through the formation of concentrated shear zones associated with localized failure. Again, the integrated buoyancy of a root section can be lowered as a result which allows negative slab buoyancy to dominate. The end result is recycling of root material. A Brittle Word or Two: Oceanic lithosphere is efficiently recycled despite having a high viscosity because it can fail in a brittle mode. If this was not the case the Earth would likely be a single plate planet like Mars or Venus. All our simulations prescribe a yield stress to oceanic lithosphere that allows it to fail in a brittle mode which allows us to model plate like behavior within a mantle convection framework. If yield stress is related to pore fluid effects and if cratonic lithosphere is relatively dry then it may have a high relative yield stress and this may provide it with stability and longevity. The simulations below explore this possibility. The yield ratio is the brittle yield stress of cratonic components relative to bulk mantle. Root only simulations only increase the yield stress of root material while root+crust models increase the yield stress of cratonic crust and deeper lithosphere. For the former situation, detachments can form in the lower crust which decreases the longevity potential of root material. The same was true for simulations in which only crust was given a high yield stress although then detachments formed just below the crust. The simulations which give the full cratonic lithosphere a relatively high yield stress can achieve almost complete stability and longevity for yield ratios of three or more. A Combined Word or Two & A Mobile Belt Word or Two: The simulation above has a root that is chemically buoyant and has a viscosity 10 times that of the mantle at equivalent temperature. The yield stress of cratonic crust and root is twice that of the mantle and the yield stress of peripheral mobile belts is one half that of the mantle. None of these individual factors is set to a sufficient value so as to be able to provide for root longevity and craton stability on their own. Together, however, there is more than sufficient synergy between the various factors to allow for stability and longevity over several mantle overturn times. By systematically varying the strength of each factor we could determine which where the more dominant. For these simulations the high relative yield stress of cratonic crust and root material was the dominant effect. The low yield stress of peripheral mobile belts means that the model craton was buffered to a degree from subduction related stress as the mobile belts could only transmit a maximum stress equal to their yield stress. This individual effect could not provide full stability, as shown in the mobile belt section at the bottom center of this poster, but it could lower the cratonic yield stress needed to provide for stability and root longevity. from Hoffman 1987 CRATON: A Region of the Continental Crust that has Attained Stability and has been Little Deformed for a Prolonged Period Tomography of Van der Lee & Nolet 130 km depth CRATON: A Continental Region Associated with a Relatively Thick Lithosphere (~ 200-300 Km) at Present K1 depth (km) 0 50 100 150 250 200 K2 K3L1 Mobile Belt Deep Structure Deduced from Xenolith Studies CRATON: A Continental Region Underlain by a Thick Lithospheric Root Composed of Chemical Light Material that Resists Subduction and Provides Protection for the Continental Crust Overlying It. Mantle Convection with Visco-Yielding Continents chemically real light material - crust chemically light material - root failed regions coldhot mantle cold viscosity 10 Pa s hot viscosity 10 Pa s 25 21 crust has own rheology root has own rheology yielding occurs when local stress exceeds yield stress set by intersection of Byerlee & viscous curves - effective viscosity in yielded regions is low and decreases with strain base of thermal lithosphere continental lithosphere is cool & more viscous than bulk mantle Mobile Belt/Craton Yield Ratio normalized root extent 0 Myr 50 Myr mobile belt width = 335 km mobile belt width = 221 km Normalized Root Extent Craton/Mantle Yield Ratio Root & Crust; 50 Myr Root & Crust: 100 Myr Root Only; 50 Myr Root Only: 100 Myr coherent removal mode almost at full craton stability - a top to bottom stability not bottom to top Normalized Root Extent Log Root/Mantle Viscosity Ratio Weak Lower Crust; 50 Myr Weak Lower Crust; 100 Myr Strong Lower Crust; 100 Myr Very Weak Lower Crust; 100 Myr coherent removal mode slice and dice removal mode coherent removal mode stretch it into bite-sized bits removal mode Ref Root  (kg/m ) 3 310033003180314032203260 Buoyancy Ratio Normalized Root Extent Root Depth=120 km; 100 Myr Root Depth=80 km; 50 Myr Root Depth=120 km; 50 Myr Root Depth=80 km; 100 Myr Buoyancy Ratio A Final Word: The essence of a craton has generally been sought in the buoyancy and/or viscosity of deep cratonic roots. Our simulations suggest that it may be more shallow, i.e., in the brittle yield properties of cratonic lithosphere. This has testable implications. A key test is that the effective elastic thickness of cratons, determined from gravity and topography, should not be much greater than 50 km. Early estimates of elastic thickness where in the 150 km range, which favored a deep essence, but more recent estimates, using improved data and methods, are falling in the 50-ish km range. The lower portions of roots should also show some signs of deformation. Seismic anisotropy studies should be able to test this shallow essence prediction. ABCDE THE ESSENCE OF A CRATON A. Lenardic - Rice UniversityL.-N. Moresi & H. Muhlhaus - CSIRO A Little More on Set Up: To show the results from a number of simulation sets in a consistent way we need to define a measure of craton stability and root longevity. Normalized root extent is used for this. It represents the lateral extent of a cratonic root. It starts at one and root recycling/deformation causes it to drop below one. Thus, simulations that maintain a value of one over time are doing better in root preservation and craton stability. Root material is 1000 X as viscous as bulk mantle at equal temperature low yield stress mobile belts Reference mantle density=3300 kg/m Reference root density =3200 kg/m 3 3