Abstract

The Hurricane fault, and other fault zones of Cenozoic age, are the result of extension in a variety of directions within a rotating system. Of these, ENE fractures are oldest, at about 20 million years. The next younger set of fractures trends NW-NNW, and the latest set, including the Hurricane fault, trends NNE. Important fault zones are superimposed upon key elements of the Sevier overthrust belt. The Hurricane fault follows the hinge line of the Sevier leading edge fold, the Kanarra anticline, from Anderson Junction to near Cedar City. The ramp fault of the Kanarra anticline may also be involved in extension during the Cenozoic. Using balancing techniques on fault- propagation folds, adjacent segments of the anticline vary in amount of shortening and depth-to-detachment. NW-trending lineaments cross the fold hinge where shortening and depth-to-detachment patterns change rapidly. These accommodation or transfer zones, formed in the late Cretaceous, are the locus of mid- to late-Cenozoic extension faults that trend NW.

Oblique action within the leading edge fold includes en echelon patterns along a small thrust on its eastern limb. In a single cross-section, this thrust zone may appear as two thrusts, about 1,000 feet apart, bounding a zone of fault chaos. The nearby Hurricane fault also has a similar double fault zone with internal chaos, suggesting en echelon patterns and an oblique action, typical of inherited structures.

The well-known reverse drag associated with the listric shape of the Hurricane and other extensional faults can also be studied using balancing principles. In extensional structures, as well as in compressional ones, balancing analysis can estimate depth to basal detachment. In many cases, this depth is similar to or is a significant fraction of the width of the surface disturbance. Dip values, measured on the tilted strata of the disturbed zone, are direct indicators of the amount of extension or compression on the listric fault. In both cases, direction of dip of beds within a fault block is useful in determining wether the block is downthrown.