Geometry of the Trachyte Mesa Intrusion, Henry Mountains, Utah: Implications for the Emplacement of Small Melt Volumes Into the Upper Crust

The Trachyte Mesa intrusion is one of several small satellite bodies to the larger intrusions of the Henry Mountains, Utah. Most previous studies have worked under the assumptions that Trachyte Mesa is blister shaped and intruded into flat and gently NW dipping strata. In this study we combine structural and geophysical data sets to constrain the structural geology of the host lithologies and the unmodified geometry of the intrusion. Trachyte Mesa intrudes a series of northeast trending upright and open folds formed within the Jurassic Entrada Formation. Truncation of these folds at the contact with the overlying Curtis/Summerville formations indicates the folds are Middle Jurassic. Magnetic and 2-D resistivity surveys focused on the southwestern portion of the intrusion where it is concealed by overlying strata. These data clearly delineate the outline of the buried intrusion. The intrusion is 2.2 km long and 0.7 km wide with an average thickness of similar to 15 m (maximum similar to 40 m). The majority of the intrusion (both exposed and buried portions) is confined within the axis of a syncline bound to the NW and SE by anticlines. The intrusion does, however, overtop the hinge of the bounding anticline to the northwest in a few places along its length. In cross section the intrusion is characterized by concave-up top and bottom surfaces, except along portions where it overtops the bounding anticline. The geometry and structural position of the Trachyte Mesa intrusion suggest that preexisting structure and the density of the magma relative to that of host rocks fundamentally controlled the emplacement of this intrusion.