EMPLACEMENT MECHANISMS, TIMING, AND INTERNAL FLOW CHARACTERISTICS OF THE INTRUSIVE SHEET NETWORK ON THE SOUTHERN MARGIN OF MOUNT HILLERS, HENRY MOUNTAINS, UTAH

The southern margin of the Mount Hillers laccolith, Henry Mountains, Utah, displays an intricate network of sills and dikes intruding tilted Mesozoic sedimentary strata. Dike-fed sills intruded into subhorizontal strata; tilting followed during growth of the underlying main igneous body, possibly accompanied by radial dike intrusion. These mid-Tertiary (~25 Ma) plagioclase-hornblende porphyry intrusions lack a syntectonic fabric overprint and cooled rapidly in the upper 2-3 kilometers of the crust to preserve fabrics resulting from magma flow. Coupled with exceptional 3-d exposure, this field area is ideally suited to study shallow crustal magma transport.

Evidence of multiple generations of magma sheet emplacement is preserved in the field: younger, fine-grained dikes nearly always cross-cut older, coarse-grained sills. In addition to cross-cutting relationships, results from crystal size distribution analysis, thin section petrography, and whole rock major and trace element geochemistry all suggest the sills and dikes are distinct magma pulses.

Field-measured fabrics in the igneous rock, while occasionally well-defined, are generally weak and difficult to measure confidently. Consistent fabric results were derived using anisotropy of magnetic susceptibility (AMS). AMS results show broadly consistent flow paths in both dikes and sills, in which foliation strikes generally subparallel to the mapped length of the intrusion.

Field observations suggest mechanical heterogeneity of host rock is important in how intrusions move through the shallow crust. Sills commonly utilized preexisting weaknesses like bedding planes to intrude, and tend to be thicker in shale than in sandstone host rock. A consistent radial pattern of dikes in the area suggest these younger intrusions propagated along radial fractures in the host rock, perhaps during the growth of the main igneous body.

Sills and dikes are the primary means through which magma moves in the shallow crust. These sheet intrusions feed plutons and batholiths that are significant components of continental crust. Networks of sills and dikes also comprise the internal plumbing of volcanic systems. Therefore, understanding how these magma sheets flow and form improves our understanding of continental crust construction, volcanic hazards, etc.