The laccolith-stock controversy: New results from the southern Henry Mountains, Utah

Abstract

Domes of sedimentary strata at Mount Holmes, Mount Ellsworth, and Mount Hillers in the southern Henry Mountains record successive stages in the growth of shallow (3 to 4 km deep) magma chambers. Whether the intrusions under these domes are laccoliths or stocks has been the subject of controversy. According to G. K. Gilbert, the central intrusions are direct analogues of much smaller, floored intrusions, exposed on the flanks of the domes, that grew from sills by lifting and bending of a largely concordant overburden. According to C. B. Hunt, the central intrusions are cylindrical stocks, sheathed with a zone of shattered sedimentary rocks, and the small flanking sills and laccoliths grew laterally as tongue-shaped masses from the discordant sides of these stocks. New geologic mapping demonstrates that the sedimentary overburden, now partially eroded from the domes, was uplifted about 1.2 km at Mount Holmes, 1.8 km at Mount Ellsworth, and at least 2.5 km at Mount Hillers. The radii of the domes are similar, between 5 and 7 km. The strata over the domes have a doubly hinged shape, consisting of a concave-upward lower hinge and a concave-downward upper hinge. A limb of approximately constant dip joins these two hinges and dips 20° at Mount Holmes, 50° to 55° at Mount Ellsworth, and 75° to 85° at Mount Hillers. The distal portion of each dome is composed of a gently dipping peripheral limb 3 to 4 km long, presumably underlain by sills and minor laccoliths. Although geologic cross sections and regional aeromagnetic data for the three domes are consistent with floored, laccolithic central intrusions, these data alone do not rule out the possibility of a stock at depth. At Mount Hillers, paleomagnetic vectors indicate that tongue-shaped sills and thin laccoliths overlying the central intrusion were emplaced horizontally and were rotated during doming through about 80° of dip. This sequence of events is not consistent with the emplacement of a stock and subsequent or contemporaneous lateral growth of sills and minor laccoliths. Growth in diameter of a stock from about 300 m at Mount Holmes to nearly 3 km at Mount Hillers, as Hunt suggested, should have been accompanied by considerable radial shortening of the sedimentary strata and a style of folding which is not observed. Geologic and geophysical data and mechanical models support a laccolithic origin for the central magma chambers underlying the domes.