ALBERT

Description: We combine heat flux data and seismic velocity models for the North American lithosphere to derive constraints on thermal conditions and deformation mechanisms in the underlying convecting mantle. Local heat flux averages that are not affected by shallow crustal heat production contrasts allow calculation of reliable lithospheric geotherms and uncertainty ranges. For consistency with the seismic data, the mantle potential temperature beneath North America must lie within a 1290°C–1450°C range, close to that for the oceanic mantle sampled at mid-ocean ridges. The heat flux at the base of the lithosphere varies laterally from 11 ± 3 mW m−2 beneath the ∼250 km thick Archean core of the Superior province to 15 ± 3 mW m−2 beneath the thinner younger Appalachians province. It is shown that the most likely cause of such rates of heat supply into the North American continent is small-scale convection in an unstable boundary layer beneath the rigid mechanical lithosphere. This allows useful constraints on the mantle rheological properties. We show that the most likely deformation mechanism is dislocation creep in wet mantle rocks. Ranges for the mantle temperature, water content, and rheological parameters could be tightened very significantly once strong constraints are obtained on radiogenic heat production in the lithospheric mantle.

Description: How continental lithosphere responds to tectonic stresses and mantle convective processes is determined in large part by its mechanical strength and temperature distribution, which depend on crustal heat production. In order to establish reliable crustal and thermal models for the Superior craton, Canadian Shield, new measurements of heat flux and heat production in 28 deep boreholes at 16 sites are combined with a larger set of older data. The Superior Province was assembled by the docking of volcanic/plutonic and metasedimentary terranes and continental fragments to the southern margin of an older core around 2.7 Ga. The average heat flux is much lower in the craton core than in the accreted terranes, 31 versus 43 mWm -2 . The major accreted volcanic/plutonic belts share the same heat production characteristics, testifying to the remarkable uniformity of crust-building mechanisms. The marked difference between the crusts of the core and the accreted belts supports the operation of two different crust-forming processes. The crust of the craton core has an enriched upper layer, in contrast to that of the younger belts which lack marked internal differentiation. At the end of amalgamation, the lithosphere of the craton core was colder and mechanically stronger than the lithosphere beneath newly accreted material. Surrounding the craton core with weaker belts may have ensured its stability against tectonic and mantle convection perturbations. This large strength contrast accounts for the lack of lithospheric imbrication at the edge of the craton core as well as for the different characteristics of seismic anisotropy in the lithospheres of the craton core and the younger terranes.

Description: Laboratory experiments document the post-emplacement behaviour of mafic intrusions that spread at a density interface and founder as they become denser than their surroundings due to cooling and crystallization. All else being equal, the larger the intrusion volume, the farther the intrusion can spread and the smaller its aspect ratio is. The final aspect ratio is a function of a single dimensionless number analogous to the Rayleigh number of thermal convection. Once it is denser than its surroundings, the intrusion becomes unstable and may founder in two different regimes. At aspect ratios larger than about 0.4, the “teardrop” regime is such that the intrusion thickens in a central region, developing the shapes of a funnel and a pendant drop. At lower aspect ratios, another regime is observed, with thickening of the intrusion at the leading edge and thinning in a central region. The thick outer ring in turn becomes unstable into a set of teardrops and leads to an irregular horizontal outline. In one variant called the “jellyfish” regime, the thin central region develops a number of downwellings and upwellings in a Rayleigh-Taylor-like pattern. These instabilities may get arrested due to cooling as the intrusion and encasing rocks become too strong to deform. One would then be left with a funnel shaped residual body or a wide irregular one with thick peripheral lobes and a thinner central region. These different patterns can be recognized in upper crustal mafic intrusions.

Description: [1]  The long geological history of passive margin evolution is complex yet typified by an initial ramp-like tilting of the subaerial surface towards the continent-ocean boundary, followed by episodic uplift and subsidence at a smaller wavelength. We argue that this behaviour is due to changes in margin structure brought about by buoyancy-driven lithospheric flow. Continental lithosphere is melt-depleted, buoyant and thick. It will resist convective breakdown into the asthenosphere below, but will be prone to lateral flow due to horizontal density contrasts. Changes in lithosphere thickness at the transition between continent and ocean will nucleate convection cells. Using a numerical model of viscous upper mantle flow we show that stability or instability of the continental lithosphere at a passive margin is a function of the lithospheric rheology and composition. Increased compositional buoyancy leads to ocean-ward lateral flow of the continental lithosphere whereas decreased buoyancy has the opposite effect, causing landward lateral flow of the continental lithosphere. In model simulations, a continental lithosphere thought typical of Phanerozoic continental platforms experiences first a margin-wide ramp-like tilting, followed by topographic fluctuations due to an evolving array of convection cells in the mantle. The timing and magnitude of predicted changes in topography are similar to those observed at the eastern North American margin, suggesting that the tilting and episodic uplift and subsidence at continental passive margins are a natural consequence of the evolution of continental lithosphere after break-up and during mature sea-floor spreading.