IGNEOUS EMPLACEMENT PROCESSES IN THE SHALLOW CRUST: BUCKHORN RIDGE INTRUSION, HENRY MOUNTAINS, UTAH

The Buckhorn Ridge intrusion is a tongue-shaped sill emplaced during mid-Tertiary time as part of the laccolithic Mount Holmes intrusive center in the Henry Mountains of southern Utah. This plagioclase-hornblende body intrudes subhorizontal sedimentary strata and its three-dimensional shape is exceptionally well exposed. The intrusion has a maximum thickness of ~60 m near its emergence from dike-like feeder conduits and thins progressively until its distal termination ~1300 m away. The intrusion bifurcates near its termination to include both a concordant lobe and a lobe that cuts up-section before again becoming concordant.

Fabric measurements of phenocryst shape-preferred orientation in the intrusion, from field data and anisotropy of magnetic susceptibility, are weak but persistent throughout the body. Foliation is sub-parallel to shallowly dipping upper and lower contacts. Lineation displays a restricted fanning pattern in the direction of magma flow.

Weak fabric development within the intrusion may be related to magma rheology. During emplacement the melt was relatively silica-rich (>60% SiO₂) and included abundant phenocrysts (>50% vol.). Consequently, magma viscosity was quite high. Emplacement-related deformation was therefore likely concentrated near the margins of intrusion, much of which may have been transported outward as a high-viscosity plug accommodated by shear near the margins of the body.

Much of this emplacement-related strain was likely partitioned into a cataclasite layer up to ~1 m thick that is spectacularly well exposed along much of the base of the intrusion. Presumably, the thin cataclasite layer accommodated most of the deformation during emplacement. This allowed the vast majority of the magma to be emplaced nearly passively and to develop very weak fabric. No cataclasite was found within the ~500 m most proximal to the presumed feeder zone. One explanation for development of cataclasite in the more distal portion of the intrusion is progressive acceleration of sill propagation rate with increasing total length. This acceleration has been predicted in dynamic models and is observed in analog models of sill emplacement.