Continental lithosphere strength: the critical role of lower crustal deformation

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Summary A thermo-rheological model of lithosphere deformation, incorporating the elastic, ductile and brittle behaviour of lithosphere material, has been used to examine intraplate continental lithosphere strength, brittle-ductile transition depth and flexural rigidity. These parameters are critically dependent on crust and mantle rheology and consequently on geothermal gradient, crustal thickness and lower crustal composition. For lithosphere subjected to a lateral tectonic force, creep in the lower crust and mantle leads to stress release, and the subsequent stress redistribution generates stresses in the upper lithosphere sufficient to cause brittle fracture. The extent of creep in the lower crust and mantle and the degree of upper lithosphere stress amplification (which together determine bulk lithosphere strength) increase with geothermal gradient. For significant lithosphere extension, under maximum likely levels of available tectonic stress, a lithosphere surface heat flow of 60 mW m −2 or greater is required, while for compressive lithosphere deformation, heat flow must exceed 75 mW m −2 . Similarly flexural rigidity increases with increase in the thermal age of the lithosphere at the time of loading. The depth of the brittle-ductile transition decreases with increase in geothermal gradient. For a limited range of gradients (expressed by heat flow q = 50–55 mW m −2 ) multiple brittle-ductile transitions may exist in the middle and lower crust and upper mantle, with important tectonic implications (e.g. for intra-crustal detachments and crust-mantle decoupling). Lithosphere strength in extension and compression, and flexural rigidity, are both controlled by the quartzo-feldspathic rheology of the crust for thermally young lithosphere and by the olivine rheology of the mantle for older lithosphere. Lithosphere strength is therefore critically influenced by the thickness of the crust (decreasing with increase in crustal thickness) and by the composition of the lower crust, particularly for lithosphere with intermediate heat flows.