ABSTRACT

Two models for the genesis of the Tendoy thrust are that it is a low-angle thrust or an upthrust. Recent field study of the upper and lower plate within one to two miles on either side of the thrust outcrop supports the former with three lines of evidence: (1) Systematic shallowing of thrust dip with increasing elevation (i.e. structural position) typical of an upthrust does not occur. Dips taken along strike at different elevations show several reversals of a shallowing trend, (2) at Straight Creek, where the thrust dips 60°, the upper plate deformation is characterized by overturned and asymmetrical folds indicating layer-parallel compression. This is incompatible with upthrust experiments and theory that show generally layer-parallel extension throughout the upthrown block, (3) the angle between the fault and lower plate bedding is everywhere less than or equal to 30° regardless of present thrust dip This suggests a low angle implacement of the fault.