Abstract

The geometry of the Rocky Mountain trench extensional fault system in northwest Montana can be defined by using, as reference features, pervasive asymmetric cleavages associated with the preexisting Lewis thrust. These Laramide fabrics act as preextensional markers of regional dip and elevation, and thus deformation of these passive markers can be used to determine the specific hanging-wall geometry of each extensional fault.

Two types of extensional fault are distinguished: (1) planar faults with tilt-block geometrics that cut through the Lewis thrust, and (2) curved faults with rollover geometrics that reactivate the existing Lewis thrust. The deformed Laramide markers indicate that the Rocky Mountain trench system is dominated by a series of tilt blocks upon which the preexisting thrust had only a localized effect.

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