EXTENSIONAL FAULTS ASSOCIATED WITH THE TWO-STAGE EMPLACEMENT OF THE TRACHYTE MESA INTRUSION, HENRY MOUNTAINS, UTAH

The igneous intrusions of the Henry Mountains, Utah, are the type locality for ‘laccolith’ intrusions; the term ‘laccolite’ coined by Gilbert in the late 1800s. The Henry Mountains are located in SE Utah on the Colorado Plateau. The range trends N-S and consists of 5 principal peaks, each representing an intrusive complex. The intrusions are mid-Tertiary in age, post-dating minor Laramide orogenic activity, and were emplaced during a period of tectonic quiescence. As a result, many of the observed structures in the host rocks to the intrusions can be attributed to the magmatic events. Likewise, fabrics within the intrusions largely reflect magmatic flow.

The Trachyte Mesa intrusion, the most distal satellite intrusion to the Mt Hillers complex, is an elongate intrusion trending NE-SW, concordant with the Entrada Sandstone it intrudes. The intrusion is comprised of multiple, stacked intrusive sheets. The excellent 3D exposures which cross-cut both intrusion and host rock make this an ideal place to study emplacement-related deformation and emplacement mechanisms. Field mapping and detailed structural data collection studies were focused on the southern end of the NW margin of the Trachyte Mesa intrusion as this is where the host rock-intrusion contact is best exposed in outcrop. Steep dip-slip (extensional and reverse) faults are observed at the tip/ termination of intrusive sheets. Offsets on bedding planes and slickenfibres are preserved on the calcified fault surface and show dominant down to the NW fault kinematics. The throw is similar to the thickness of the intrusive sheet it is associated with. Mapping of these faults along strike, away from the exposed contact zone, reveal an arcuate trend that appears to match the previously proposed curved nature of the ‘lobe’/promontory of stacked intrusive sheets emanating from of the main intrusion. Magma can also be seen intruding along some of these faults: this “sill-climbing” is good evidence that this extension is syn-emplacement. Extensional strain parallel to the intrusion margin is consistent with a two-stage growth for the emplacement mechanism, with the magma spreading laterally as a thin sill, then inflating upwards, jacking up the host rock along brittle normal faults.