The geometry of brittle-ductile extension and modes of normal faulting during steady or episodic rifting of the continental lithosphere are investigated by analogue modeling using the centrifuge technique. We use a frictional material to simulate the brittle behavior and various nonlinear bouncing putties to represent the ductile rheology of the stratified lithosphere. Steady stretching of the continental lithosphere supports rapid differential subsidence and flexure of the brittle crust with formation of marginal grabens and a panel of domino-style fault blocks. In contrast, episodic stretching of the lithosphere (progressing in a series of pulses separated by longer periods of quiescence) supports formation of spaced grabens and horsts, with less subsidence. This is due to a counterflow of ductile material from the margins to the central rift zone. We show here that the isostatic adjustment in the lithosphere can be related to the steady or episodic mode of heterogeneous stretching. We apply the model results to the southern Red Sea, for which the inferred opening history involved continuous or stepwise extension.

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