Concentration of shearing deformation related to changes in strike of monoclinal fold axes: the Waterpocket monocline, Utah

Abstract

Detailed field investigation of the Waterpocket monocline within the Utah portion of the Colorado Plateau reveals a strong correlation between abrupt changes in the trend of the monoclinal fold axis and enhanced fault development within the Navajo Sandstone. Faults within the bends of the fold axis range in scale from individual deformation bands to large, complex fault zones visible on 1:40,000 scale aerial photographs. Slip vectors on faults within the bends examined range from nearly pure strike-slip to subequal amounts of strike-slip and dip-slip offset. The spatial density and size of individual faults decrease rapidly away from the fold axis bends. We interpret the abrupt variations in fold axis orientations to result from segmentation of the basement faults underlying the monocline or similarly oriented bends in the basement faults. In this regard, the structures observed within the Navajo Sandstone are transfer structures developed above relay zones of the basement fault segments. The exact nature of the deformation at a given bend depends upon the geometry and kinematic history of the fault segments at depth. Variations in the proportion of strike-slip and dip-slip offset on these faults suggest that variables such as the orientation of the bend with respect to the direction of maximum compressive stress during the Laramide and details of the geometry of adjacent basement fault segments may form important controls on the nature and distribution of secondary structures in the folded strata. The results of this study highlight the need to consider along-strike variations in fold geometry in addition to considering the effects of cross-sectional fold shape when assessing fold-related fracturing.