TRIANGLE ZONE GEOMETRY, TERMINOLOGY AND KINEMATICS

The term ''triangle zone'' was first used informally in 1957 to descri be the structure of the east edge of the Foothills thrust belt of the southern Canadian Cordillera. Since then, triangle zones have been rec ognized at the foreland margins of most of the world's thrust-and-fold belts. Variations in terminology of triangle zones reflect difference s in interpretation of their internal geometry and their role in oroge nesis. The principal issue is whether triangle zone formation occurs o nly at the final stage of evolution of a thrust belt or is a continuou sly regenerated tectonic process that creates deformed belts. Evidence is given to support the second interpretation, leading to a hypotheti cal kinematic model for the formation of thrust-and-fold belts. Accord ing to this model, emplacement of successive triangle zones creates a foreland-migrating frontal monocline. Physiographic and structural rel ief of this structure continuously regenerates a foreland-migrating en velope of overpressured section in the undisturbed foreland sequence a head of it, causing continued progradation of the deformation front. I n some areas gravity sliding takes place down the foreland-facing dip slope of frontal monoclines. The gravity slides themselves may have em ergent or buried (triangle zone) thrust fronts.