Significance of conjugate folds and crenulations in the central Sierra Nevada, California

Abstract

Conjugate folds and crenulations occur in various parts of the central Sierra Nevada. Analysis of individual conjugate folds shows that the principal directions of compression associated with these structures were reasonably constant over a large area during the conjugate folding. Analysis of the modal axial plane distribution of the accompanying crenulations and small folds produces comparable results, suggesting that the direction of maximum compression may be reliably determined from modal data. Both methods of analysis indicate that the axial planes of small folds and crenulations associated with the conjugate set have formed at a moderate angle (generally 40° to 60°) to P max . In morphology, the crenulations vary from isolated kinks to a crenulation cleavage. Locally this cleavage develops into a differentiated crenulation cleavage and in some outcrops takes on many of the characteristics of a penetrative slaty cleavage. These structures have formed at approximately 40° to 60° to P max . It is proposed that slaty cleavage can develop in two dynamic frameworks: (1) where slaty cleavage forms at 90° to P max , and (2) where cleavage forms at 40° to 60° to P max . Analysis of the direction of maximum compression shows that during the conjugate folding, P max was oriented approximately parallel to the structural grain of the Sierra Nevada. This implies a shift in P max of nearly 90° between development of the principal slaty cleavage and conjugate structures. Similar shifts in P max between development of slaty cleavage and conjugate folding are observed in other mountain belts. We propose a model that explains this shift in P max by suggesting that the formation of slaty cleavage and subsequent crenulations represents structures formed at different stages of development within a continuous process. This process involves compression normal to the mountain belt (formation of slaty cleavage), and as normal compression ceases, “elastic recovery“ becomes active, inducing shortening parallel to the belt (formation of conjugate crenulations). The model implies negligible time lapse between formation of slaty cleavage and subsequent crenulations.