GEOMETRY OF THE TRACHYTE MESA INTRUSION, HENRY MOUNTAINS, UTAH: IMPLICATIONS FOR THE EMPLACEMENT OF SMALL MELT VOLUMES INTO THE UPPER CRUST

The Trachyte Mesa (TM) intrusion is one of several small satellite bodies to the larger intrusions of the Henry Mountains, Utah. Several previous studies have worked under the assumptions that TM 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 unmodified geometry of the intrusion. Trachyte Mesa intrudes a series of northeast-trending upright and open folds formed within the Jurassic Entrada Fm. Truncation of these folds at the contact with the overlying Curtis/Summerville Formations indicates the folds are Middle Jurassic. Magnetic and 2D 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 geometry of the intrusion is 2.2 km long, 0.7 km wide with an average thickness of ~15 m (max. ~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. These observations suggest that the preexisting structure (i.e. folds) had a fundamental role during the emplacement of the TM intrusion.