Conjugate crenulation cleavages in the Uncompahgre Formation, Needle Mountains, Colorado

Abstract

Multiply deformed metasediments of the Proterozoic Uncompahgre Formation contain a well-developed conjugate set of crenulation cleavages and related folds. Conjugate character is indicated by the presence of conjugate folds and crenulations, synchronous cleavage development, lack of consistency in sense of overprinting where overprinting exists, and bimodal attitude distribution on πS 2 diagrams. Despite the overall conjugate character of the folds and cleavages, most outcrops exhibit only one of the two possible sets. Attitudes of fold hinges and intersection lineations suggest that these conjugate structures formed late in the second phase of deformation, after extensive D 1 transposition and after D 2 macroscopic folds were well-developed and cut by fold-related thrusts. Although considerable controversy exists regarding whether non-conjugate sets of crenulation cleavages develop parallel to the XY plane of the strain ellipsoid or not, it is clear that conjugate crenulation cleavages must develop at an oblique angle to the XY plane. Although individual outcrops may develop one member of a conjugate set to the exclusion of the other, mechanical and geometric constraints necessary for conjugate cleavage development appear to be such that it is extremely unlikely that only one member of a conjugate set would develop over an entire region to the total exclusion of its conjugate pair. Although similar in appearance, conjugate and non-conjugate crenulation cleavages appear to be two different species, one forming at a substantial oblique angle to the XY plane, the other probably forming parallel or nearly parallel to it.