Small-scale models of multiring basins

Small-scale sand-silicone simulations of multiring impact structures have b een undertaken in order to understand the effects of the rheology of the li thosphere on the variability of natural multiring structures. For low sand- silicone thickness ratio (1:3), brittle strain is accommodated by spiral st rike-slip faults. For higher sand-silicone ratios (1:1 or 2:1), an inner co ncentric ring affected by strike-slip faults is relayed by an external ring affected by concentric normal faults. The diameter of the inner ring decre ases with the increase of the sand-silicons thickness ratio. It is suggeste d that the flexure of the brittle layer due to the silicone now is responsi ble for the brittle strain field which is enhanced by the channel flow of t he lower crust. The characteristic geometry of the intersection of conjugat ed strike-slip faults can be observed around large multiring basins on sili cate crust such as Orientale on the Moon and on icy crust, such as Valhalla on Callisto and Gilgamesh on Ganymede. The strain field around these large craters is discussed in terms of mechanical properties of the lithospheres . On the Moon, large craters without relaxation faults, such as Imbrium are located on thin crust regions. The crust was too thin to have a ductile lo wer layer at the time of impact. Gilgamesh on Ganymede is surrounded mainly by strike-slip faults. Asgard on Callisto has the same diameter as Gilgame sh but is surrounded by concentric normal faults. The brittle-ductile thick ness ratio is thus higher on Callisto than on Ganymede.