Effects of Initial Weakness on Rift Architecture Dyksterhuis, S., Rey, P, Muller, D. and Moresi, L. Abstract Much work has been conducted investigating the primary factors controlling rift architecture, using both computational and laboratory methods.  Here, we examine the effects and relative importance that different types of initial weaknesses have on extensional lithospheric deformation style.  We find that the type of initial weakness included in the model plays a primary role in determining the subsequent rift mode. Models using single localised weak seeds produce symmetric narrow rifts, irrespective of whether or not strain-softening is included, while models that include regions of diffuse weaknesses produce a wide rift mode.  Models that include an initial weak fault tend to produce more asymmetric rifting leading to a core complex mode. By distributing the strain laterally over large areas, the ductile lower crust tends to oppose the narrow rift mode forced by the initial weakness. These results suggest that initial weaknesses may play a major role in determining the mode of rifting. Our numerical experiments confirm that low angle faults can form as the result of rotation of initially high angle faults. While previous studies have suggested that the rheological and thermal profiles of the lithosphere play the most important role in rift mode determination, our results illustrate that initial weaknesses could play a major role in rift mode determination, highlighting the need to make initial weaknesses a primary consideration when modelling the extensional deformation of the lithosphere.

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Coupled Models	Decoupled Models
Single Seed Weakness: Upper Crust Lower Crust Mantle Diffuse Random Noise High Angle Fault	Single Seed Weakness: Upper Crust Lower Crust Mantle Diffuse Random Noise High Angle Fault

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Single Seed Weakness

Upper Crust

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Lower Crust

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Mantle

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Diffuse Random Weakness

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High Angle Fault Weakness

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