Mechanisms of thrust propagation: some examples and implications for the analysis of overthrust terranes

Abstract

Evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move. The mechanisms are: (1) failure of a stiff layer, to form ramp-flat geometry (‘imbricated’ thrust sheets); (2) detachment of a layer by folding (‘décollement” thrust sheets); (3) differential layer-parallel shortening (‘LPS’ thrust sheets). The mechanisms are independent but may operate interactively to form families of ‘hybrid” thrust sheets. Since LPS thrust sheets have not been previously described or documented, data from the New York plateau, deformed almost exclusively by differential LPS, is presented to demonstrate the physical characteristics of this thrust mechanism. Information from the plateau indicates that finite-strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial. Finite-strain data are used to construct a set of iso-strain maps in both the deformed and undeformed states. The iso-strain maps are in turn used to determine the displacement field for the thrust sheet. The displacement field allows visualization of the effects of both Lagrangian and Eulerian transformations on an initially orthogonal grid. Utilizing sections from a number of overthrust belts, it is shown that the three mechanisms occur universally although in various proportions. LPS is regionally developed throughout the central and northern Appalachians. The presence of the LPS thrust sheets probably accounts for the failure of structural cross-sections to bed-length balance in these blind thrust terranes. Finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross-sections.