LARAMIDE STRUCTURE OF THE UNCOMPAHGRE PLATEAU OF WESTERN COLORADO

The overall Laramide-age structure of the Uncompahgre Plateau of western Colorado is a broad, NW­-SE striking crustal arch cut by a complex oblique-slip fault system. WNW-ESE striking, subvertical, left-lateral oblique-slip faults are the most common structures and form distinct northern and southern zones.

In the northern zone, multiple WNW-ESE striking, left-lateral, oblique-slip faults splay into transpressive bends along multiple NW-SE striking monoclines and reverse faults (this includes the classic Redlands Monocline of the Colorado National Monument). Evidence for left-lateral strike-slip along the controlling WNW-ESE striking, oblique-slip faults includes: (1) fault slickenline data; (2) the presence of right-lateral strike-slip faults with a conjugate NE-SW strike; (3) occurrence of curving swarms of N-S to NE-SW striking faults in Mesozoic sandstones that we interpret to be right-lateral R’-shears within an overall zone of left-lateral shear. In addition to left-lateral strike-slip, these faults have a component of dip-slip that is always down-to-the north, but the sense of dip-slip changes along strike from normal to reverse. This is because throughout their length, these faults have subvertical dips that changes or ‘wobbles’ along strike (over distances of 100’s of meters) from either steeply north-dipping (normal dip-slip) to steeply south-dipping (reverse dip-slip).

Along the southern Uncompahgre Plateau, well-developed, WNW-ESE striking, left-lateral oblique-slip faults also occur and are best developed along the antiformal crest of the Uncompahgre crustal arch. In this southern zone, the left-lateral oblique-slip structures are only associated with transtension; the sense of dip-slip is always normal and there are no monoclines. The location of many of these faults along the crest of the Uncompahgre crustal arch suggests that Laramide faulting in this area was influenced by local extension in the outer arc of a synchronously developing basement arch. These transtensional faults were also more open to fluid migration resulting the in widespread development of hydrothermal breccias, synkinematic mineralization and extensive silicification of permeable sandstones adjacent to these faults.