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Abstract

Contrasting styles of lithospheric extension are investigated with two-dimensional plane-strain finite-element models in which the brittle and duc-tile behavior of lithospheric rocks are represented, respectively, by elastoplasticity and thermally acti-vated power-law viscoelasticity. Parameter values were chosen to agree with laboratory measure-ments.

Emphasis is placed on a comparison between the postcompressional extension of tectonically thick-ened lithosphere and the extension of normal thick-ness lithosphere containing a weak zone. The former may represent extension of the Basin and Range province, whereas the latter may correspond to extension at a simple rifted continental margin.

Models of the collapse of a region of tectonically thickened lithosphere, like Tibet, under forces from its own excess potential energy (f) and from in-plane stress transmitted horizontally through the lithosphere (f) predict that the strain rate and total extension are sensitive functions of its thermal state. Three time scales are critical: (1) the time available for thermal equilibration of the tectoni-cally thickened lithosphere before the compressive stress is removed; (2) the time that f3 acts alone; and (3) the time that f and f3 act together. Extension is maximized when thermally equilibrated lithosphere collapses and rupture can then be achieved by the addition ofanf tensile force which is as small as 20 MPa for the parameter values chosen. By contrast, normal lithosphere which has been weakened by a factor of 100 requires f = 90 MPa to cause rupture.

Inherited tectonic characteristics, like the initial distribution of thickened lithosphere, and the three time scales may govern the extensional style. Both pure shear and “pinch-and-swell” styles are pre-dicted, and large-scale simple shear occurs when laterally offset weak zones are present. Many of the differences between the style of Basin and Range extension and that of typical rifted margins may be attributed to tectonic inheritance, as can the strike variability of rifted margins. If the Basin and Range province was similar to Tibet before extension, caution is required in applying models that are appropriate for this tectonic setting to typical continental margins which may have very different initial lithospheric properties.

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