Field-Based Constraints on Rates of Dynamic Igneous Emplacement Processes in Subvolcanic Systems, Henry Mountains, Utah

Eric Horsman, East Carolina University
Michel de Saint Blanquat, CNRS / Université Paul-Sabatier
Sven Morgan, University of Michigan - Dearborn
Scott Giorgis, SUNY - Geneseo
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Abstract

We use careful analysis of spectacular 3-d exposures of ancient igneous intrusions and host rock to provide insight into the dynamic deformation processes, including their rates, operating in subvolcanic environments. The Buckhorn Ridge intrusion in Utah's Henry Mountains is a tongue-shaped sill emplaced during the Oligocene as part of a sill / dike complex overlying the laccolithic Mount Holmes intrusive center. This dioritic, plagioclase-hornblende porphyry sill intrudes shallowly dipping sedimentary strata and its full 3-d shape is exceptionally well preserved. 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 exceptional 3-d exposure of the sill provides a framework to estimate numerous characteristics of intrusion emplacement.

Magma driving pressure calculations suggest emplacement at a depth of less than 1-2 km, consist with constraints from thermochronology, mapping, and reconstructions of Oligocene stratigraphy. Paleomagnetic analysis of the diorite suggests it was emplaced after the underlying, unexposed main laccolith had already made space for itself by uplifting and rotating host rock. We estimate a conservative maximum duration of emplacement for the 0.02 km^3 sill of 1 year. This estimate is based on the minimum required flux through a dike network, which is controlled by several parameters, most notably magma viscosity (melt viscosity calculated from geochemical data and adjusted to account for phenocryst abundance). True emplacement duration was likely only a few weeks.

Weak but consistent fabrics (both field-measured and magnetic) defined by phenocryst alignment are developed throughout the sill. The weak fabric development is likely related to partitioning of emplacement-related strain into phenocryst-poor intrusion margins and host rock. Calculations suggest phenocryst-poor (~5-10% by volume) intrusion margins had magma viscosity approximately five times lower than the adjacent phenocryst-rich (30-35% by volume) intrusion interior. Partitioning of strain into low-viscosity sill margins would also result in localized shear-related heating of these margins, promoting a positive feedback loop. Emplacement-related strain produced a marginal breccia zone of igneous and sedimentary host rock up to ~1 m thick. Spectacular exposures of this breccia zone exist at the sill tip and along much of the base of the intrusion. Estimates suggest the sill tip propagated at least meters per day and that strain rates in the marginal zones exceeded typical experimental rates of 10^-5 to 10^-6 per second. But dominant processes and their rates varied widely on short spatial and temporal scales.

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