Gravity sliding, major thrusting, and the displacement of large crustal plates are accomplished because strata with the lowest viscosity in a rock sequence readily deform by simple (rectilinear) shear flow when subjected to loading. These deforming strata are called décollement zones. Strata above these zones are transported without being deformed, provided their viscosity is one or more orders of magnitude larger than that of the décollement zone. Shear stresses acting at the base of these strata, that is, “basal drag,” are commonly much lower than stresses accompanying friction across a sole fault (compare with Hubbert and Rubey, 1959). Thus, shear flow in décollement zones most likely represents the operative mechanism in natural settings. The speed of gravity sliding and the ease of tectonic transport increase with increasing décollement zone thickness, dip, and depth of burial, and decrease with increasing décollement zone viscosity. End effects, including (in two dimensions) updip attachment of gravity slides and frontal buttressing of both slides and tectonic plates, are as important as basal drag in determining the speed of slides and the resistance to transport of crustal plates.
Structures in the strata above a décollement zone develop independently of those within the zone. In the upper plate, pull-apart grabens form to overcome updip attachment; step thrusts and folds form to overcome frontal buttressing. Structures within décollement zones are proposed to include intensely developed passive disharmonic folds, bedding plane faults at contacts between high and low viscosity strata and through high viscosity masses within the zone, chaotic zones comprised of blocks of high viscosity strata, infolds of adjacent high viscosity strata, and penetrative cleavage, phylonitization, and other features of deformation-induced recrystallization.
Velocity profiles for simple shear portray the general mode of deformation and permit study of transport in specific geologic settings. Composite velocity profiles are constructed by first drawing individual profiles for each rock in the sequence under study. Segments from these curves proportional to the thickness of each succeeding stratum are then spliced together from the bottom up.