ENERGY BUDGET DURING FOLD-TIGHTENING: AN EXAMPLE FROM THE CANYON RANGE SYNCLINE, CENTRAL UTAH, SEVIER FOLD-THRUST BELT

Cores of first order folds often hold critical clues to an entire folding history. Fold shape adjustments often begin in the fold core and continue to take place there because space problems predominantly arise in that region. The core of the Canyon Range (CR) syncline, part of an internal thrust sheet in the central Utah segment of the Sevier fold-thrust belt (FTB), underwent fold tightening within the elastico-frictional (EF) regime, chiefly by fracturing and cataclastic flow. Under these deformation conditions, several stages of the fold's history are well preserved; younger fractures simply cross-cut older fractures rather than erase evidence for previous stages of deformation. The core of the CR syncline is composed of alternating competent quartzite and incompetent phyllite layers (Cambrian Pioche unit). Using cross cutting relationships, we have tracked the behavior of the competent and incompetent layers during folding and determined when/where they interacted with one another. This deformation history is used to develop a kinematic model of fold tightening. The CR syncline initially folded essentially by pure bending processes and then continued to tighten predominantly by buckling.

In addition, we used the kinematic history of the CR syncline to analyze fold tightening processes in terms of energy consumption; based on this approach, the following questions are addressed: (1) what fold shape provides the most energy efficient geometry (e.g. rounded vs. angular hinge), (2) do similar fold shape adjustments take place in both buckle and bend type folds, (3) how much energy is consumed within different rheologies in different parts of the fold (hinge vs. limb), (4) does the most energy efficient geometry change during different stages of fold tightening, and if so how, (5) once a fold reaches a certain interlimb angle, do buckling and bending processes break down.

Since most folds are produced by a combination of buckling/bending processes and are composed of competent and incompetent units, this analysis of the CR syncline has far reaching applicability to other folds.