THE OLDMAN RIVER TRIANGLE ZONE - A COMPLICATED TECTONIC WEDGE DELINEATED BY NEW STRUCTURAL MAPPING AND SEISMIC INTERPRETATION

New structural mapping and seismic interpretation indicate that the tr iangle zone and related structures exposed along and adjacent to the O ldman River, southern Alberta Foothills, comprise a complicated struct ure involving significant deformation in the hanging wall of the upper detachment. This deformation includes orogen-directed thrusts and lar ge orogen-vergent folds (ca. 1 km amplitude) as well as apparent secon dary orogen-directed tectonic wedging. Although duplex structures are interpreted to occupy the interior of the triangle zone, the term ''pa ssive-roof duplex'' is inappropriate due to the degree and nature of d eformation above the upper detachment. The upper detachment is best vi ewed as a structural domain boundary, separating dominantly foreland-d irected structures below from dominantly orogoen-directed structures a bove. Multiple levels of detachment have resulted in two ''nested'' te ctonic wedges which show evidence for some synchronous deformation. Ou r interpretation suggests that a relict triangle zone toe may be prese rved and exposed along the Oldman River, immediately above the princip al upper detachment (the Big Coulee Fault).