Mechanical stability of the triangle zone: The backthrust wedge

Abstract

The triangle zone arrangement of faults that commonly develops at the foreland margin of fold‐and‐thrust belts is one of the few places in this structural setting that backthrusts of significant dimensions are observed. Even in this setting, the backthrust may be a relatively ephemeral feature, as it can be cannibalized by the foreland advance of thrusting in an active fold‐and‐thrust system. One of the factors that determines the longevity of a specific backthrust and triangle zone in an active system is the mechanical stability of the backthrust wedge (the wedge of rock that forms the hangingwall to the backthrust). Upper and lower stability limits for this backthrust wedge can be determined using the critical wedge concept for a cohesive Coulomb material. The configuration of the backthrust wedge developed in triangle zones along the southern and central Alberta portion of the Canadian Cordillera agrees well with the stability limits derived for reasonable rock properties and normal fluid pressures. In contrast, a low‐taper triangle zone interpretation for northeastern British Columbia does not fall within the calculated stability limits, even when abnormally high fluid pressures and extreme rock properties are assumed. However, a similar low‐taper geometry may be achieved by the shingling of stable backthrust wedges. Upper crustal delamination and passive‐roof duplexing are both processes that serve to sustain a mechanically stable backthrust wedge configuration, suggesting that they are both viable paths for the progressive advance of the mountain front triangle zone.