BALANCING THE MONTANA ROCKY MOUNTAIN TRIANGLE ZONE: WHAT TO DO ABOUT LAYER-PARALLEL STRAIN?

The Rocky Mountain triangle zone defines the NE edge of the Alberta salient of the Rocky Mountain fold-and-thrust belt for 1300 km, from NE British Columbia to central-western Montana. In Canada, the triangle zone is a major producer of hydrocarbons, but in Montana it is inadequately explored. The triangle zone comprises an antiformal duplex wedge with a triangular cross-section in weak foreland-basin strata. It is best understood through the construction of balanced cross-sections. The roof strata are parautochthonous, having been tilted toward the east, but not significantly shifted in the horizontal direction. The tilt reversed the gentle westerly dip of the foreland basin strata, defining a synclinorium that includes the Alberta, Augusta, and Adel synclines. Most of the structures in the adjacent Foothills belt are east-verging folds and imbricated thrusts with significant cumulative horizontal displacement that was accommodated by simultaneous movement along the roof and floor faults of the triangle zone wedge. Balanced and restored cross-sections indicate that the cumulative displacement of the Rocky Mountain fold-and-thrust belt increases systematically from a few km near the NW and SE tips of the Alberta salient to 250 km at the mid point. Yet the triangle zone is continuous and has similar structural relief and cross-sectional geometry along its entire length. This indicates that the triangle zone was a steady-state, sub-cutaneous structure that evolved in the van of the fold-and-thrust belt. Foreland basin strata were cannibalized from the flank of the triangle zone as it progressed. Along the Sun River of Montana, calcite vein systems and strained burrows in Campanian Two Medicine Formation sandstones indicate that triangle zone strata experienced significant NE-SW layer-parallel shortening and NW-SE extension and fluid expulsion prior to buckling and faulting. After section balancing and restoration of thrusts and folds, allowance must be made for the layer-parallel strain for full evaluation of bulk shortening.