SANDBOX EXPERIMENTS AS ANALOGUES FOR CONTINENTAL RIFTS: IMPACT OF ROCK STRENGTH IN EXTENSIONAL STRESS REGIMES

Experimental sandbox analogs are an effective method for studying incremental strain at the scale of the continental crust. We investigate the role of rock strength on the nascent stages of continental rifts, such as the East African Rift. Normal faulting in the first experiment (using only sand) resulted in the development of horst and graben features. The second experiment included a strong stiff layer; strain was accommodated by block rotation and localized along normal faults dipping on average 86¢ª. The third experiment included a weak layer that progressively thinned, showing listric boarder faults that changed dip as a function of depth, with average dip angles of 32¢ª (ductile faulting). The fourth experiment modeled a stiffer upper crust overlying a deeper ductile layer. Deformation in the stiff layer occurred through a combination of fracturing, faulting, and block rotation. Dip angles of normal faults were on average 84¢ª in the brittle layer and 30¢ª in the ductile layer. Graphical representations of basin width and depth as a function of extension show that basin width increased more in experiments involving a ductile layer, whereas basin depth increased more in experiments containing a brittle layer. Calculated beta values of rotated blocks show greater rotation in the center of the rift, with an average β value of 1.7 in experiment two and 1.3 in experiment four. Analysis using Mohr circle diagrams show differences in strain as a result of varying materials. The results demonstrate that rock strength variation within the crust impacts both the mechanisms and distribution of strain throughout continental crust undergoing extension. Further information can be found on the Wikispaces page named Rock Trauma Center at http://rocktraumacenter.wikispaces.com/Sandbox+Models.