| Abstrakt: |
Faults in the western part of the Colorado Plateaus and the adjacent eastern part of the structural transition zone to the Basin and Range in central Utah represent two stages of deformation: Laramide compressional faults on the plateau (eastern) side and late Cenozoic extensional faults on the transition-zone (western) side. The faults range in displacement from a few centimeters to several hundred meters. This study of minor structures provides strong evidence for regional NE-SW horizontal maximum compressive stress as a dynamic component of monoclinal flexure; the results further suggest that it is inaccurate to interpret the monoclines simply as passive drape folds formed during vertical block up-lift. The Laramide faults on the plateau side cut two types of large-scale folds: (1) a broad warp called the Teasdale anticline, and (2) two monoclines that flank the Teasdale anticline, called the Waterpocket and Cocks Comb monoclines. The faults that cut the monoclines dip steeply and have mostly strike-slip displacements. Strike azimuths of dextral and sinistral faults tend to occupy separate sectors, suggesting that they are conjugate shears activated in a stress field with a subhorizontal maximum compressive stress (σ1) oriented northeast-southwest. This orientation is confirmed by computations of mean paleostress characteristics by inversion of fault-slip data. The monoclines differ from one another in trend by 30°, so it is possible to compare these trends with the results of separate paleostress computations for faults from each monocline. Four separate computed σ1 axes (three from the Cocks Comb monocline) are approximately horizontal and cluster within 10° of the N. 65° E. normal to the axial trace of the Waterpocket monocline, suggesting that the Waterpocket at the latitude of our study formed approximately perpendicular to the local Laramide compressive-stress direction. The maximum compressive stress (σ1) is located clockwise about 30° from the normal to the Cocks Comb monocline. We interpret this discordance as evidence that the Cocks Comb trend was influenced by, and in large part inherited from, the trend of an ancient buried fault rooted in the basement, whereas the exposed faults from which σ1 was computed are structures that formed in, and are probably restricted to, the sedimentary cover rocks. The exposed faults probably formed fresh in the cover rocks during Laramide compression and are generally more reliable indicators of compressional kinematics than fold orientations. Part of the Cocks Comb monocline shows a 13° distortion of its axial trace by cumulative dextral slip on hundreds of small-displacement transverse faults. We interpret this distortion as a late-stage, brittle compressional strain (20%) related to the buttressing by mechanically resistant rocks in the most structurally elevated part of the adjacent Teasdale anticline. Fault-slip data were gathered from the Thousand Lake fault, the easternmost main fault zone of the transition zone. Faults that parallel the main fault show mainly dip slip, but many faults in the zone strike oblique to it and show strike slip or dip slip. Analysis of the predominantly dip-slip faults indicates a west-northwest/east-southeast (285°) least principal stress/strain axis that we interpret as the neotectonic extension direction for the area. [ABSTRACT FROM AUTHOR] |
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