Lower crust indentation or horizontal ductile flow during continental collision?

Conditions for indentation and channelised flow are investigated with two-dimensional thermomechanical models of Alpine-type continental collision. The models mimic the development of an orogen at an initial central portion of weakened lithosphere 150 km wide, coherent with several geological reconstructions. We study in particular the role of lower crustal strength in developing peculiar geometries after 20 Ma of shortening at 1 cm/year. Crustal layers produce geometries of imbricate layers, which result from two contrasted mechanisms of either channelised ductile lateral flow or horizontal rigid-like indentation: - Channelised lateral flow develops when the lateral lower crust has a viscosity less than 10²¹ Pa s, exhibiting velocities opposite to the direction of convergence. This mechanism of deformation produces subhorizontal shear zones at the boundaries between the lower crust and the more competent upper crust and lithospheric mantle. It is also associated with a topographic plateau that equilibrates with a wide (about 200 km) but quasi-constant crustal root about 50 km deep. - In contrast, indentation occurs with lateral lower crust layers that have a viscosity greater than about 10²³ Pa s, producing significant shortening and thickening of the central crust. In this case topography develops steep and narrow (around 100 km wide), associated with a thickened crust exceeding 60 km depth. A crustal-scale pop-up forms bounded by subvertical shear zones that root into the mantle lithosphere.