ABSTRACT

Upheaval Dome (Canyonlands National Park, Utah) is an enigmatic structure previously attributed to underlying salt doming, cryptovolcanic explosion, fluid escape, or meteoritic impact. We propose that an overhanging diapir of partly extrusive salt was pinched off from its stem and subsequently eroded. Many features support this inference, especially synsedimentary structures that indicate Jurassic growth of the dome over at least 20 m.y. Conversely, evidence favoring other hypotheses seems sparse and equivocal.

In the rim syncline, strata were thinned by circumferentially striking, low-angle extensional faults verging both inward (toward the center of the dome) and outward. Near the dome's core, radial shortening produced constrictional bulk strain, forming an inward-verging thrust duplex and tight to isoclinal, circumferentially trending folds. Farther inward, circumferential shortening predominated: Radially trending growth folds and imbricate thrusts pass inward into steep clastic dikes in the dome's core.

We infer that abortive salt glaciers spread from a passive salt stock during Late Triassic and Early Jurassic time. During Middle Jurassic time, the allochthonous salt spread into a pancake-shaped glacier inferred to be 3 km in diameter. Diapiric pinch-off may have involved inward gravitational collapse of the country rocks, which intensely constricted the center of the dome. Sediments in the axial shear zone beneath the glacier steepened to near vertical. The central uplift is inferred to be the toe of the convergent gravity spreading system.

ON THE COVER: Meteorite impact, salt structure, or...? Upheaval Dome, in Canyonlands National Park of Utah, looking west-northwest from the air (photograph by John S. Shelton). The Green River canyon is at the top. From the margins of the photo, layers pass through an extended zone and the rim syncline ino the constricted and uplifted core of the structure (inset at lower right). These features, as well as varied synsedimentary structures, salt welds, and lack of impact-related products, argue for an origin by necking of the stem of a broadly overhanging salt extrusion, as discussed in "Structure and evolution of Upheaval Dome: A pinched-off salt diapir" by Martin Jackson, Daniel Schultz-Ela, Michael Hudec, Ian Watson, and Michael Porter.

Published in Geological Society of America Bulletin, December, 1998, pp. 1547-1573.