Abstract

An area in southwestern Middle Park, Colorado, has been studied in the field. Mapped principal structural features include the Williams Range thrust and a major normal fault east of the thrust. Direct observation of a cross section of the thrust surface, data on its elevation and its trace, an observation of its dip, and an observation on rotation of strata in the overblock show that the thrust surface is curved, being nearly horizontal in its westernmost exposures with progressively steeper eastward dips to the east. The mapped structural features postdate deposition of the Pierre Formation, and the thrust is known to have experienced recurrent movement.

This paper explains the development of the mapped principal structural features in terms of a system of stresses acting on the boundaries of the area. Because east-west cross sections are similar, an explanation is offered for an average cross section of, specifically, height equal to 6500 feet and length equal to 32,500 feet. Analysis is treated as a two-dimensional problem. The system presented consists of a combination of two systems of stresses. One is characterized by a step displacement of the midpoint of the lower boundary, the other by a cosine displacement of the lower boundary. The analysis assumes that stress within the body is describable by methods of theory of elasticity and that faults and stress are related by the Mohr criterion. The analysis also assumes that faulting does not affect the orientation of principal stresses as described at the instant prior to inception of faulting and that it tends to increase the magnitudes of stress near its tip and decrease them where faulting has transpired.

The system accounts for the attitude of faulting in the average cross section and predicts fault development where faulting has occurred. The explanation is compatible with lesser structural features of the overblock of the area, including high-angle faulting at the north and synclinal development in the youngest Tertiary rocks between the thrust and major normal fault. The explanation is compatible with the regional structural geology and is geologically practicable.

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