Numerical modeling of a laterally heterogeneous lithosphere under extension: Strength contrasts and rifting of a metasomatized craton
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The heterogeneous composition of the lithosphere affects the way it deforms under extension. This thesis investigates the effects of heterogeneities that introduce compositional, thermal, and thickness contrasts within an extending lithosphere using 2D-numerical thermal-mechanically coupled finite element methods. Rifting in the absence of large quantities of melt (magma-poor rifting) is the result of the initiation and growth of necking instabilities. Two main factors that determine the growth rate of necking instabilities are the brittle vs. viscous (approximately stiff vs. pliable behavior) of lithospheric layers and the rate of deformation (strain rate) applied. In a laterally homogeneous lithosphere the stiff vs. pliable behavior determines the location of localization and the timing of rifting, because the strain rate is equally distributed laterally throughout the layers and does not preferentially enhance growth of any individual necking instability. On the other hand, where a lateral contrast is introduced the strain rate is no longer equally distributed throughout the layers and this may alter the location of lithospheric breakup to the location of the necking instability under the highest strain rate. The first part of the thesis investigates the competition between growing necking instabilities in a vertically layered lithosphere with models where a vertical lithospheric boundary is present. The strength of the crust and mantle changes across this vertical boundary and creates a strain rate contrast. Localization occurs in the higher strain rate lithosphere in most model configurations. Only where the background strain rates are near equal can localization in a stiff layer overtake necking instabilities in layers with higher strain rates. The results are applied to: 1) the formation of asymmetric rifted margins where strain localizes at the boundary between contrasting lithospheres, but deformation is mostly distributed throughout the lithosphere under a higher strain rate, and; 2) the preservation of strong cratons, where they are protected by surrounding younger and weaker lithospheres affected by a higher strain rate. In the second part of the thesis, more complex models, with thermal, compositional, and thickness contrasts, are employed to expand on the second application and show that craton rifting requires significant weakening of the cratonic lithosphere. Weakening of the craton is accomplished by melt metasomatism. Melt injected into the cratonic mantle lithosphere effectively increases the density from 3335 kg m-3 to 3378 kg m-3, increases the temperature from the heat released by melt as it cools and crystallizes, and decreases the viscosity by rehydration. Especially important is the increase in temperature, which decreases the viscosity and strength of the craton so that it becomes weaker than the surrounding lithosphere. In particular, a craton can be rifted when its mantle lithosphere is thinned and heated just below the Moho. In this case, the craton is weaker than the younger lithosphere, the roles are reversed, and the craton protects the younger lithosphere from rifting. The heterogeneity introduced in the models presented throughout this thesis illustrates the complex way in which the lithosphere is affected by inherited structures and contrasts, and provides a new understanding of the role these heterogeneities may have in altering the location of localization in an extending lithosphere.