Styles of deformation in the ramp regions of overthrust faults

Abstract

The presently available and widely used model describing the style of deformation of rocks in ramp regions of overthrust faults is that proposed by Rich (1934) for the Pine Mountain overthrust in the southern Appalachian Plateau. The model implies that (1) rocks in the upper plate migrate over and past a ramp by continuously folding and unfolding, (2) the attitude of the ramp is paralleled by the total thickness of the upper plate in the ramp region, and (3) the configuration of upper plates in ramp regions is constant regardless of variation in rock types, sequence of rock types, or bedding thickness. Field observations of several well-exposed ramp regions of small-scale overthrusts do not support these implications, but rather reveal a variety of deformation styles. There are at least three modes by which upper plate rocks move over and past ramps in the fault surface. Mode 1 involves imbricate thrusting in the lower, trailing edge of the ramp. The ramp segments of these imbricates dip less steeply than the main ramp. This mode is observed only where thickness of individual beds or mechanical units in the upper plate are small compared to the height of the main ramp. Mode 2 involves a thickening, in the lower ramp region, of the upper plate unit which is in contact with the sole fault. Movement takes place along the sole fault, and along the upper contact of the thickened unit. In both modes, the effect of the deformation is to move material into the lower portion of the main ramp and to create a shallower ramp over which the upper-plate rocks can, presumably, more easily move..