SHAPE EVOLUTION AND FABRIC DEVELOPMENT DURING PROGRESSIVE GROWTH OF SHEETED IGNEOUS INTRUSIONS, HENRY MOUNTAINS, SOUTHERN UTAH

The Tertiary igneous intrusions of southern Utah's Henry Mountains provide an ideal location to study igneous emplacement processes. Exceptional exposures allow us to constrain three-dimensional intrusion geometry and to study emplacement-related fabric in detail. We examined three neighboring intrusions in this area: a sill, an elongate laccolith, and a bysmalith (a cylindrical intrusion that uplifts its overburden in a piston-like manner by slip along a curved fault). Our results suggest that (1) the three intrusions represent successive stages in the progressive growth of a small igneous body, (2) each of these intrusions was emplaced through the progressive amalgamation of multiple igneous sheets, and (3) changes in fabric between intrusions reflect changes in geometry and emplacement mechanism.

Fabric within the sill records the relatively early stages of emplacement. The sill is composed of two distinct, vertically stacked igneous sheets. Additionally, in map view the sill has a complex geometry consisting of an elliptical main body from which protrude several elongate finger-like lobes, which may represent the distal edge of a propagating sheet. Both the fabric within and geometry of the elongate laccolith record a more protracted history of sheeted emplacement. During progressive emplacement, sheets are stacked atop one another as they bend overlying sediments to locally create space for the intrusion. In the bysmalith, the fabric record of the earlier sill and laccolith stages is locally preserved, but much of the fabric formed during vertical inflation of the intrusion. Whether this inflation occurred through the amalgamation of many separate sheets is unclear from fabric alone; contacts between sheets are cryptic.

Our results demonstrate that the roll of sheeting as an emplacement mechanism evolves as intrusion geometry evolves from sill to laccolith to bysmalith. Additionally, the fabric record of emplacement processes can be complex and may not record the entire history of the intrusion's construction. In particular, evidence of sheeted emplacement processes may locally become cryptic as intrusions grow in size and form relatively long-lived magma chambers large enough to modify some early formed fabrics.