ABSTRACT

We investigate oblique slip in external zone (foreland) thrust systems by deforming analogy models constructed of plasticine and silicone putty in the Experimental Tectonics Laboratory at Queen's University. Horizontally laminated forelands were shortened by gravitational collapse and spreading of a hinterland wedge of plasticine, the frontal edge of which had an oblique strike relative to the direction of gravitational spreading (O = 15°, 30°, and 45°).

Models contain either a single strong unit over a weak unit, or two strong units overlying two weak units. The models deform by thrusting, folding, layer-parallel shortening (LPS), and orogen-parallel extension (OPE). Thrusts are mostly foreland-propagating, and crop out at the top (free) surface in two-unit models.

By tracking particle displacement paths on the free surface we demonstrate that the hanging walls of thrusts were displaced primarily updip relative to their immediate footwalls (fixed), with a lesser component of orogen-parallel displacement in zones of high strain that are associated with thrusts. Thus, the net thrust displacement is oblique, but this is not revealed by serial-sections cut parallel to the boundaries of the models. Oblique displacement and OPE are increased when space is provided for strike-parallel motion of material.

Volume is conserved in the centrifuge, making three-dimensional balancing of a set of serial cross-sections routine. However, restoration of the sections requires a quantitative knowledge of fault displacement, and partitioning of the strain mechanisms OPE, LPS, folding and thrusting. Our results suggest that many natural external zone thrust faults may have a concealed component of orogen-parallel displacement.

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