Formation of dome-in-dome structures: Results from experimental studies and comparison with natural examples

Dome-in-dome structures are frequent in salt provinces and in high-grade rocks of orogens. Their origin is only poorly understood. In the present study we have modelled dome-in-dome structures, which result from constrictional (radial) in-plane shortening of a competent layer embedded in a rising less competent matrix. Both the layer and the matrix consist of non-linear viscous plasticine, which display a viscosity contrast of 5. The deforming competent layer is characterized by (1) a striking downward drag along the margins, (2) a sample-scale first-order dome, and (3) numerous second-order domes and basins straddling the first-order dome. With increasing layer thickness and strain, the amplitude, A, arc-length, L, and wavelength, λ, of the second-order domes and basins increase. However, their growth rate is significantly smaller than that of non-rising domes and basins, which result from a bidirectional flow. This restricted growth of rising domes and basins is attributed to the simultaneous growth of the first-order dome, and explains, why hair-pin type folds – typical for non-rising domes and basins - are lacking, even at higher finite strain. Similar dome-in-dome structures like those produced in the present study have been described from deeper levels of salt and gneiss domes or from foliation triple points of interfering diapirs.