Field mapping studies will be used to evaluate and apply the critical wedge model of thrust sheet mechanics in an attempt to resolve existing conflicts between thrust sequences predicted by the model and those suggested by geological evidence. According to the critical wedge model, the advance of the thrust belt under the force of horizontal compression, requires that the wedge deform internally to attain and maintain a critical taper angle. Potential taper-building mechanisms are: out-of-sequence thrusting; duplex formation; and synchronous thrusting. Field work began in 1988 and 1989 in the Sawtooth Range of the northwest Montana thrust belt during a previous award entitled "Sequence of Thrusting: Implications for Mechanical Models of Thrust Belts". Existing geological maps proved to be inaccurate or inadequate for the purpose of detailed structural analysis and the area had to be remapped. Two additional field seasons will be required to completed the mapping phase. The construction of regional cross-sections, sequential restoration of the sections, and forward modelling, will answer two principal questions: (1) which of the taper-building mechanisms were operative? and (2) have they occurred in sufficient magnitude to have allowed the critical wedge model to function?
