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Geological Society, London, Special Publications; 1981; v. 9; p. 55-63;
DOI: 10.1144/GSL.SP.1981.009.01.06
© 1981 Geological Society of London

I. Mechanics of Thrusts and Nappes

Thrust sheet deformation at a ramp: summary and extensions of an earlier model

D. V. Wiltschko

Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

A previous model for the equilibrium of a linearly viscous thrust sheet at a ramp shows that fault-zone drag is the most important energy sink, though bending can be an important resisting force and gravity can be a moderate driving force. The model was developed for the Pine Mountain block of the southern Appalachians, though it is general and may be adapted to other geometries.

Increasing the viscosities, ramp dip and thickness of the sheet each impedes forward motion whereas increasing the dip of the erosional surface and fraction of incompetent material and including the effect of an overriding thrust sheet aid movement. Doubling the viscosities doubles the bending resistance. Doubling the block thickness increases bending resistance as well, but since the force required to overcome this added resistance acts over twice the surface area, the net traction is reduced. Doubling the ramp dip greatly increases bending resistance, but if the length of the ramp is adjusted to maintain the same stratigraphic climb, this increase is lessened. The dip of the erosional surface is linearly related to the traction due to gravity in the ‘glacier analogy’ model. However, gravity cannot overcome all other sources of resistance for a sheet of the dimensions of the Pine Mountain block unless the surface slopes at 16° in the direction of transport. Increasing the fraction of incompetent material decreases bending stresses and therefore reduces the net traction necessary for movement. Finally, in the case of an overriding thrust sheet, if the upper fault zone is equal in shear strength to the lower fault zones, the resistance to movement of the lower one is cancelled; only bending resists motion. Successively overriding thrust sheets is a documented kinematic scheme in major thrust belts and the resulting applied shear stresses may be one mechanism by which stress and displacement are transferred from higher to lower sheets.