Folding by cataclastic flow at shallow crustal levels in the Canyon Range,Sevier orogenic belt, west-central Utah

Folds form by ductile deformation typically involving continuous Row. In th e elastico-frictional regime, such deformation may be accomplished by catac lastic flow involving collective movement on a population of fractures and zones. The Canyon Range (CR) syncline, part of the CR thrust sheet in west- central Utah, developed in this regime. The CR syncline is composed of thic k-bedded quartzite units with a small material contrast between layers, lim iting limb rotation by flexural slip alone. Thus, fracture populations deve loped to accommodate fold tightening by limb rotation and thinning, and the formation of transverse zones across the fold. Several generations of frac ture and deformation zone (DZ) networks are recognized from mesoscopic and microscopic evidence, and can be related to stages of folding. The net resu lt of the large number of distributed fractures and deformation zones is a continuous deformation that is homogeneous at the scale of the outcrop. All these lines of evidence suggest that large-scale cataclastic flow accommod ated folding by allowing rigid mesoscopic blocks to slide along bounding DZ s. Along its length, the CR syncline consists of several segments bounded by t ransverse zones with different mechanisms accommodating fold tightening in adjacent segments. In one segment, fold tightening progressed by limb rotat ion, and then out-of-the-core thrusting. In contrast, fold tightening in th e adjoining segments occurred by rotation and thinning of one limb and poss ible hinge migration, with the steeply dipping to overturned limb showing p rogressive thinning of units on a megascopic scale and progressive increase in the thickness and density of deformation zones at all scales. (C) 2001 Elsevier Science Ltd. All rights reserved.