Internal deformation in clay models of extension by block faulting

Clay and sand have provided useful analogues for the failure by block faulting of continental crust under extension. We performed experiments on clay models of such faulting and found that the total horizontal displacement on the major faults provided only 40–50% of the known extension. Similar experiments on sand showed that the displacement provided 70–80% of the known extension. Examination of the differential tilt in both models also underestimated the extension. The hidden extension is due to internal deformation within the blocks. Assuming that the sand and clay deform exclusively by slip and rotation on four families of faults, and that these faults have many scales, we propose that the hidden extension occurs on faults below the visible scale. We observed much of the deformation in sand because the grain size, the lower limit for faulting, was close to the minimum observable scale for the model. On the other hand, in clay, two or three orders of faults lay below this scale, giving rise to a hidden extension of 50% or more. We cannot apply our results directly to the block faulting behavior of continental crust. However, our method of analysis suggests an approach that may have value. Deformation within the continental crust occurs at many scales. Some of these scales remain hidden when determining the amount of extension. In such cases, our postulated behavior for clay may present a useful analogue as it suggests that deformation within the blocks may play an important role. For example, estimates of extension found by summing the horizontal displacement on normal faults observed on multichannel seismic lines are lower than those given from analyses of the subsidence of many sedimentary basins and continental margins. Hidden deformation at a scale below that observable on the seismic line could account for this difference.