Lithosphere Temperature Constraints and the Thermal Control of Lithosphere Thickness

English Abstract:

The lithosphere thickness, as defined by the boundary between conductive and underlying convective asthenosphere heat transport, is directly related to the thermal regime. Although local variations, thermal regimes and thicknesses are strongly bimodal; cold strong cratons (Can. Shield) with thicknesses of 200-250 km and hot weak backarcs (Cordillera) with thicknesses of about 60 km. Intermediate thicknesses appear to be transient. I first summarize the temperature constraints, and then discuss time variations in lithosphere thickness and their possible origins. Good temperature-depths require more than one estimator; the three principal are: (1) Heat flow-heat generation; upper crust radioactive heat is important; increasing temperature uncertainty with depth. (2) Mantle seismic velocity; temperature is the primary control of mantle velocity; some composition effects. (3) Upper mantle xenoliths; rocks torn off the conduit walls by kimberlites in cratons and volcanics in bacarcs that preserve insitu P-T. Continents divided into: (1) Hot backarcs like Cordillera (<50 Ma); (2) Cold cratons like Canadian Shield (thermotectonic age >500 Ma); asymptotically approach adiabatic below 200-250 km. (3) Transient conditions: Intermediate (50-500 Ma) cooling and thickening from hot backarcs following subduction termination (e.g., Appalachia); and second, rapid lithosphere thinning from cold-thick to thin-hot backarc regimes (e.g., Colorado Plateau). Some questions: Lithosphere thickness and the thermal regime are directly related, but which is the primary control? Why are craton lithospheres thick and backarcs thin? Craton upper mantles may be refractory so convection is limited to below ~200 km, but how was a special mantle formed if the lithosphere thickens with age from a formerly hot backarc? Vigorous backarc convection may result from viscosity reduction by subducted water, but how does it start? i.e., abrupt thinning of the lithosphere?