JOINT-ZONE DEVELOPMENT IN THE NAVAJO SANDSTONE OF THE COLORDO PLATEAU: EVIDENCE FOR CORDILLERAN RELAXATION, ZION NATIONAL PARK, UTAH

Late Paleogene extensional collapse of the Cordilleran foreland is manifest by a set of half grabens (Constenius, 1996). During the late Tertiary forces arising from the development of the San Andreas transform boundary drove further extension in the southern Cordillera. Extension within the eastern central Basin and Range initiated 16-17 Ma and exhibited a WSW extension direction [Wernicke et al, 1988; Snow and Wernicke, 2000]. Although listric faulting to form half grabens is the major mechanism allowing extension in the brittle crust, jointing is evidence for modest, yet pervasive Basin and Range extension in the western margin of the Colorado Plateau.

Zion National Park (ZNP) is located at the western margin of the Colorado Plateau, ~100 km east of the major normal faults of the northeastern central Basin and Range subprovince. At ZNP the landscape is a consequence of differential weathering of the Navajo Sandstone where closely-spaced vertical joints constitute joint zones that erode to form regularly-spaced (half kilometer) slot canyons striking 171º. Between these joint zones is a set of isolated joints striking 159º. This sequence of jointing records a counterclockwise rotation of the regional extension directed WSW in the Navajo Sandstone at ZNP at the time of WSW extension in the Basin and Range.

Slot canyons sit immediately above and parallel to joint zones and exhibit a regular spacing (~ 450 m). The joint zones, in particular, consist of vertical and steeply dipping joints that tend to dip toward the axis of the canyon. These regularly spaced, joint localized canyons are confined to the Navajo, suggesting a stress shadow origin for their configuration; however, this explanation does not predict closely spaced joints in joint zones at each canyon. Preferential erosion along early NNW joints initiated the pattern of canyons with a cross-sectional profile consistent with blunt edge-cracks.