Modelling localization during simple shear deformation of the lithosphere

We study the formation of shear bands in 3D numerical experiments of welded viscoplastic layers undergoing simple shear with a small component of extension or compression. We first examine the spacing of shear bands in viscoplastic-over-viscous layers with a 2.5D geometry and compare the results with the equivalent spacing in 2D extensional models. The relationship between spacing and integrated strength of the layers is found to be similar in each case. We find 3 distinctive modes of deformation. 1) For a weak lower (viscous) layer, deformation in the upper layer quickly localizes to a single shear band. 2) As the lower layer becomes stronger - comparable with the upper layer, multiple shear bands in the upper layer co-exist due to the resistance of the lower layer to strong localization in the upper. 3) When the strength of the lower layer exceeds that of the upper, it is common for a single shear band to form in the upper layer with a near-horizontal detachment at the interface of the two layers. We also examine how the shear bands evolve and interact in full 3D simulations --- in particular how the initial orientation of incipient shear bands is inherited as they link into larger, through-going structures. Inherited structure can be amplified into significant topography as strain accumulates. We examine how the deformation evolves for each of the observed deformation modes.