Collaborative Research: Three Dimensional Magmatic Strain within Laccoliths, Bysmaliths, and Sills in the Henry Mountains, Utah

EAR-0003574

EAR-0003463

This project is designed to understand the three-dimensional fabrics and strain associated with magmatic forceful expansion by comparing simple natural and experimental examples. The project involves data collection using: 1) anisotropy of magnetic susceptibility (AMS), 2) shape preferred orientations (SPO), 3) computed X-ray tomography (X-ray CT), 4) field measurements on the magmatic fabrics and strains associated with sills, laccoliths, and bysmaliths in the Henry Mountains, Utah, and 5) lab measurements on particles within artificial flows. The Henry Mountains intrusives are Oliogocene - Miocene age and intruded the upper crust of the Colorado Plateau, which is a late Paleozoic and Mesozoic section of flat-lying sandstones and shales.

The goal is to relate magmatic fabrics to the displacement of the wallrocks caused by magmatic intrusion (inflation), and the intrusives in the Henry Mountains provide the best opportunity to understand this fundamental process. The floors, walls, interiors, and roofs of these intrusions are all very well exposed in the desert plateau of southern Utah. These plutons have raised the overburden by either bending the overburden or faulting the overburden immediately above the intrusion. The shapes of the intrusions represent a complete gradation between sills and laccoliths. By documenting the magmatic fabric in these bodies, the temporal fabric development of the intrusions is being interpreted and related to steps in the evolution of flow in the physical models.

The boundary conditions of intrusion are well understood: laccoliths, bysmaliths, and sills intruded into the undeformed, horizontal strata of the Colorado Plateau. The strata act as markers to record the translation, rotation, and internal strain caused by magma intrusion. Moreover, the plutons intruded into a part of the Colorado plateau that has not experienced regional deformation. This setting precludes tectonic forces being involved with emplacement and allows an interpretation that unambiguously describes fabrics in terms of magmatic processes.

The project involves a detailed field study to document the foliation and lineation within these intrusions. In addition, the three-dimensional SPO of the phenocrysts is being recorded and the intrusions are being sampled for AMS. The AMS technique will be corroborated by X-ray CT techniques, which will allow the documentation of the orientation and spatial distribution of magnetite on the same samples.

The field studies are being conducted in tandem with physical experiments of non-uniform flows. These intrusives are the perfect examples to be modeled since their shapes are simple and known, and the low crystal content of the magmas is well constrained. The flows that are being modeled include decelerating flow and cylindrical flow, which provide a first-order approximation of magma intruding a laccolith and a bysmalith, respectively. Passive, neutrally-buoyant markers are being placed in these experiments in order to obtain quantitative strain associated with various amounts of fluid flow into a chamber. The results are being compared with the fabrics from the Henry Mountains intrusives, which were emplaced with varying amounts of magma. Using this variety of techniques, and the ideal setting provided by the Henry Mountains, the three-dimensional fabric associated with forceful emplacement of magmatic intrusions will be understood.