C. M. Cooper

Craton formation: Internal structure inherited from closing of the early oceans

The closure of ancient oceans created a dynamic setting suitable for craton formation via the thickening of continental material over a mantle downwelling. This process subjected the thickening lithosphere to extensive deformation, forming internal structure that can be preserved over the lifetime of the craton. Recent seismic imaging of cratonic lithosphere has led to observations of anomalous features colloquially known as midlithospheric discontinuities. These discontinuities are attributed to a range of sources, including the lithosphere-asthenosphere boundary, melt accumulation, and phase transitions. However, the internal structure imaged within these cratons might be reflective of their formation. In particular, the orientation and nature of the variable depths of the midlithospheric discontinuities suggest a more complicated origin such as that which could be introduced during the formation and thickening phase of cratonic lithosphere. Here, we present geodynamic models demonstrating the internal structures produced during the formation of cratonic lithosphere as well as new seismological observations of midlithospheric discontinuities in the West African craton, together with reassessment of midlithospheric discontinuities observed in the North American, South African, Fennoscandia, and Australian cratons. We suggest that the midlithospheric discontinuities observed in these cratons could be remnants of deformation structures produced during the formation of the cratons after ancient oceans closed.

Does the mantle control the maximum thickness of cratons

Geochemical observations suggest that cratonic xenoliths originate from depths no greater than 250 km, which implies a maximum craton thickness. What determines this thickness? In general, the stability and longevity of cratons depend on their ability to resist deforming forces exerted from below by the flowing, evolving mantle. We employ an analytical approach to relate the lithospheric viscosity structure of a craton to shear tractions exerted by asthenospheric shear flow. As the net thickness of the chemical and thermal components of a craton increases, these tractions increase dramatically for non-Newtonian rheology and begin to deform the cratonic base. Thus, overly thick lithosphere is subjected to large basal stresses that weaken the thermal boundary layer and diminish its ability to buffer the craton from destructive mantle flow. This feedback prevents cratonic lithosphere from growing thicker than a maximum value. However, we show that this maximum thickness increases slightly with increasing vigor of mantle convection but decreases rapidly as convective vigor decreases. Thus, we predict relative stability of cratonic thickness during most of Earths history but instability in the future.

Inferences on Australia's heat flow and thermal structure from mantle convection modelling results

Mantle convection is important in understanding the heat flow and thermal structure of the continental lithosphere, as it produces time variability in the surface heat flow, and allows for lateral advection of heat under a continent. Many of the fundamental questions in continental heat flow depend on the magnitude and variability of the mantle contribution to continental heat flow. We summarise the current understanding of the thermal state of the Australian continent, and discuss the application of mantle-convection modelling results to continental heat-flow problems. A particle-in-cell finite-element code is used to show how the continental thermal field is modulated through time, and how the calculated mantle heat flow decreases with both root thickness and crustal heat production. An increase in root thickness is shown to enhance the stability of the deep continental thermal field. These modelling results imply a modest variation in the mantle heat flow of Australia through time, and suggest that the variation in mantle heat flow over the stable Precambrian shield will most likely be indiscernible. The thickness and thermal structure of the Australian lithosphere is, to a degree, dependent on the history of mantle convection around this continent.

Moving lithospheric modeling forward: Attributes of a community computer code

GS A TO DA Y | JU NE 20 15 C.M. Cooper, Washington State University, School of the Environment, P.O. Box 624812, Pullman, Washington 99164-2812, USA; Eric Mittelstaedt, University of Idaho, Dept. of Geological Sciences, 875 Perimeter Drive, MS 3022, Moscow, Idaho 838443022, USA; Claire Currie, University of Alberta, Dept. of Physics, Edmonton, Alberta, Canada T6G 2G7; Jolante van Wijk, New Mexico Institute of Mining and Technology, Dept. of Earth & Environmental Science, 801 Leroy Place, Socorro, New Mexico 87801, USA; Louise Kellogg, Lorraine Hwang, University of California Davis, Earth and Planetary Sciences, Computational Infrastructure for Geodynamics, 2215 Earth and Physical Sciences, One Shields Avenue, Davis, California 95616, USA; and Ramon Arrowsmith, Arizona State University, School of Earth & Space Exploration, P.O. Box 876004, Tempe, Arizona 85287-6004, USA

Formation of cratonic lithosphere during the initiation of plate tectonics

Earth’s oldest near-surface material, the cratonic crust, is typically underlain by thick lithosphere (>200 km) of Archean age. This cratonic lithosphere likely thickened in a high-compressional-stress environment, potentially linked to the onset of crustal shortening in the Neoarchean. Mantle convection in the hotter Archean Earth would have imparted relatively low stresses on the lithosphere, whether or not plate tectonics was operating, so a high stress signal from the early Earth is paradoxical. We propose that a rapid transition from heat pipe–mode convection to the onset of plate tectonics generated the high stresses required to thicken the cratonic lithosphere. Numerical calculations are used to demonstrate that an existing buoyant and strong layer, representing depleted continental lithosphere, can thicken and stabilize during a lid-breaking event. The peak compressional stress experienced by the lithosphere is 3×–4× higher than for the stagnant-lid or mobile-lid regimes immediately before and after. It is plausible that the cratonic lithosphere has not been subjected to this high stress state since, explaining its long-term stability. The lid-breaking thickening event reproduces features observed in typical Neoarchean cratons, such as lithospheric seismological reflectors and the formation of thrust faults. Paleoarchean “pre-tectonic” structures can also survive the lid-breaking event, acting as strong rafts that are assembled during the compressive event. Together, the results indicate that the signature of a catastrophic switch from a stagnant-lid Earth to the initiation of plate tectonics has been captured and preserved in the characteristics of cratonic crust and lithosphere.