Abstract

The northern Oquirrh Mountains are underlain mostly by the complexly folded and faulted Oquirrh formation (Mississippian, Pennsylvanian and Permian). Structurally, this part of the range consists of two major blocks which have been subdivided into minor blocks. The major blocks are the western Lake Point block and the eastern Magna block, separated by the Garfield fault, a north-trending, right-lateral tear fault, convex westward.

The Lake Point block is characterized by east- to east-northeast-trending folds that include a broad shallow syncline on which are superimposed several chevron folds; at the north end, tight folds, overturned toward the south, are cut cut by high-angle reverse faults. Low-angle thrusts have been mapped east of Garfield smelter and at the range front in Rogers Canyon.

The Magna block is characterized by northeast-trending folds that are mostly symmetrical, broad, and open. Many are arcuate, convex northwestward. The block is cut by high-angle faults, some of which may be tear faults; most are of small displacement.

These structural differences indicate that the folds in the two blocks were not formed in place at the same time. We suggest that the folds were formed prior to and during movement of the blocks into their present position on a major thrust fault, and that counterclockwise rotation of the Magna block on the Garfield tear fault during southward movement accounts for the differences in trends of fold axes.

The structural features in the Oquirrh Mountains were formed during a long interval, probably during several orogenic epochs. These can be divided into at least six stages; the first four stages were probably related to Laramide orogeny and the last two to Tertiary and Quaternary deformation. The stages include (1) early folding along north-south axes, closely followed by (2) west-to-east thrusting; (3) later folding along

Publication authorized by the Director, U.S. Geological Survey east-west axes was followed by (4) north-to-south thrusting and related strike-slip faulting; (5) steep normal and reverse faults formed during the next stage, during the Tertiary; and (6) range-front faults developed during late Tertiary and Quaternary time. Only the last four stages have been recognized in the north end of the Oquirrh Mountains; the first two have been identified in the southern part of the range and in the Wasatch Mountains to the east.