Kinematic implications of joint zones and isolated joints in the Navajo Sandstone at Zion National Park, Utah: Evidence for Cordilleran relaxation

At Zion National Park (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 351°. Between these joint zones is a set of isolated joints striking 339°. Fracture interaction and horsetail/wing crack development indicate that the 339° striking joint set is younger than the 351° striking joint zones, despite the lateral extent of stress reduction shadows in the vicinity of the large-scale joint zones. In addition to an older, less pervasive, ∼020° joint set, this sequence of jointing records a counterclockwise rotation of the regional extension directed from WNW to WSW in the Navajo Sandstone at ZNP. 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. Extension within the eastern central Basin and Range initiated during the Miocene and exhibited a WSW extension direction [Anderson, 1971; Wernicke et al., 1988; Snow and Wernicke, 2000]. The correlation between nearby Basin and Range extension and the extension direction for the 351° tending joint zones of ZNP is so close that the jointing at ZNP is interpreted as evidence for modest, yet pervasive Basin and Range extension in the western margin of the Colorado Plateau.