SHEET-LIKE CONSTRUCTION AND WALL ROCK DEFORMATION SURROUNDING THE TRACHYTE MESA LACCOLITH, HENRY MTS., UTAH

The Trachyte Mesa laccolith (TMl) is a small (1.5km x 0.5km) thin (<50 m) laccolith on the margin of the Tertiary Henry Mts. intrusive complex. The magma intruded into and deformed flat-lying sandstones of the Jurassic Entrada Fm. Several exposures on the margins exhibit sub-horizontal, 1-2 cm thick shear zones which demarcate individual magma sheets. Many of these individual sheets exhibit bulbous margins at their terminations. The igneous sheets only exhibit high strains at the outermost 2-3 cm and are interpreted to be plug-flows. The top of the laccolith consists of a series of 1 to 2 m high plateaus, which are also interpreted to be sheets. The one exposure (stream gorge) that cuts into the interior of the laccolith does not exhibit any recognizable contacts (fabrics), except for erosional profiles that resemble subhorizontal sheets.

Anisotropy of magnetic susceptibility (AMS) data from over 100 sites on top of the intrusion are interpreted to indicate that magma flow from the uppermost sheets spread laterally from a southwestern source and that there are two domains to the top of the intrusion. Magnetic lineations in the NE domain have a central axis parallel to the long axis of the intrusion (NE) and fan outward to both sides. Magnetic lineations from the SW domain also fan outward, but the central axis curves.

At one location on the margin, the flat-lying sedimentary country rock layers can be observed as they rotate and rise up to become the roof-rocks. Thinly bedded shales and sandstones at the contact are intensely sheared and rotated to 70°. A thick (~6 m) sandstone bed on top of the thinner beds is mildly strained and bent upward, but the maximum rotation is only to 30°. The space created between these two sets of layers is filled by late sheets which deform and protrude through the steep, thinner bedded shales and sandstones.

The field, microstructural, and magnetic data indicate that the laccolith grew vertically by stacking of subhorizontal sheets. Subsequent sheets intrude on top and cause sedimentary layers to bend upward at the margin. Resistance to bending by the strong outer layer over the steep margin of the growing intrusion produces low-pressure regions which are immediately filled by late sheets. Magma probably originated from Mt. Hillers, one of the main intrusive centers in the Henry Mts. and along strike with AMS lineations.