ROTATING AND LAMINAR MAGMA FLOW AND MULTIPLE SHEET-LIKE EMPLACEMENT INTO THE TRACHYTE MESA LACCOLITH, HENRY MTS, UTAH

The Trachyte Mesa, a small (1.3 x 0.5 km) laccolith along the flanks of the Henry Mountains was emplaced through injection of multiple sheets of magma. The intrusion is porphyritic diorite with mm scale amphibole needles and is Tertiary in age. The laccolith was emplaced into the Jurassic Entrada Sandstone within the Colorado Plateau of south central Utah. Excellent exposures allow for floors, roofs, and terminations of this laccolith to be examined and sampled for microstructures and AMS. Sheets are sill-like in that they are emplaced along horizontal bedding planes, although sheets have semi-circular terminations in map view and are more like flattened fingers in overall shape. Viewed head-on, sheets have width to height aspect ratios of 4 or 5 : 1. Sheets vary in thickness between 0.5 and 12 m. Viewed in cross section, sheets have bulbous nose-like terminations. Individual sheets grow by concentrating laminar flow along the top of the sheet which directs magma to the "bulbous nose", or termination of the sheet. Laminar flow along the top of the sheet produces a shear couple along the roof and bulbous nose, but not along the base of the sheet. At the bulbous nose, the flow rotates downward and around the bulbous nose to eventually become the base of the sheet. Therefore, sheets grow by expansion into the wall rocks of pre-existing thick sheets, not through "crack opening" from a thin sill which increases in thickness away from the crack tip. Sedimentary rocks in front of and in between sheets are deformed by intense cataclastic flow, producing a fault-like gouge. Sedimentary layers above this gouge layer are vertically translated by roof uplift over the margin of the sheet, creating space for the intrusion. Space is also locally accommodated by faulting and vertical translation. Overlying sedimentary layers are also attenuated through several different kinds of deformation mechanisms which principally accommodate a loss in porosity.