RATES AND MAGNITUDES OF STRAIN DURING IGNEOUS EMPLACEMENT 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 dioritic, plagioclase-hornblende porphyry body intrudes very shallowly dipping 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 development within the intrusion is very weak. This may be related to magma rheology. During emplacement the melt was relatively silica-rich (~63% SiO2) and included abundant phenocrysts (mean ~35% vol.). Magma viscosity was consequently quite high. However, near intrusion margins phenocryst abundance was much lower (5-10% vol.). Quantitative estimates suggest intrusion margins had magma viscosity approximately an order of magnitude lower than adjacent intrusion interiors. Emplacement-related deformation was therefore likely concentrated at intrusion margins. Much of the interior may have been transported outward as a high-viscosity plug accommodated by shear near the margins of the body.

In addition to phenocryst-poor margins, emplacement-related strain was 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 phenocryst-poor and adjacent cataclasite layers accommodated much of the deformation during emplacement. 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 before slowing due to viscous dissipation. This acceleration has been predicted in dynamic models and is observed in analog models of sill emplacement. Strain rate estimates in the marginal zones considerably exceed typical experimental deformation strain rates of 10^-5 or 10^-6 per second.