PIV ANALYSIS OF BACKSTOP EVOLUTION IN AN ANALOG (SANDBOX) MODEL OF A FOLD-THRUST BELT

The evolution of fold-thrust belts has been compared to the evolution of a sand wedge pushed by a bulldozer, as characterized by critical-taper theory. The entity that applies tectonic stress to the sand, the backstop, can be defined by the hinterland-most end of an active fold-thrust belt wedge. Sandboxes with moveable walls or floors have been used to model fold-thrust belts. Recently, Particle Image Velocimetry (PIV) techniques have been used to track the kinematics of wedge formation and the distribution of strain in fold-thrust belts. We have applied PIV to characterize the evolution of the backstop position within the wedge. Previous interpretations assume that either the backstop remains fixed at the hinterland edge of the sandbox, or that the effective backstop position, the boundary at which tectonic stress transfers to the deforming wedge, migrates toward the foreland over time. By using PIV to track the boundary between active and inactive sand, during progressive shortening of a 5 cm-thick sand layer, we show that neither of the previous interpretations adequately predicts backstop evolution. Rather, the position of the effective backstop changes throughout the formation of the fold-thrust belt. The fixed or static backstop (Kopp and Kukowski, 2003) remains largely unchanged during deformation, and is a triangular wedge of permanently stable sand adjacent to the hinterland wall of the model. New material is added to the toe of the fold-thrust belt wedge by the formation of a new forethrust-backthrust conjugate pair (FBC). The effective or dynamic backstop (Kopp and Kukowski, 2003) jumps forelandward to coincide with the backthrust surface. With continued deformation, its position migrates to the hinterland until that FBC branch line reaches the base of the static backstop. Then, a new FBC initiates at the foreland edge of the thickened wedge, and the dynamic backstop jumps to the foreland again. Our results imply that the effective backstop of a fold-thrust belt is indeed a dynamic entity that jumps to the foreland, allowing transmission of stress into new, off-scraped material, and then migrates hinterland as the off-scraped material transitions into being part of the wedge.