Modeling Intrusive Geometries of a Shallow Crustal Intrusion: New Evidence From Mount Ellsworth, Utah

Author

Nathan Nushart, University of South FloridaFollow

Graduation Year

2015

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Geology

Major Professor

Paul Wetmore, Ph.D.

Committee Member

Rocco Malservisi, Ph.D.

Committee Member

Sarah Kruse, Ph.D.

Keywords

Geology, Laccolith, Structural Geology, Volcanology

Abstract

Surface displacements resulting from upper-crustal intrusion of melt are a paramount concern for communities and facilities located in or near active volcanic areas (e.g. Campi Flegrei, Yucca Mtn.). Study of active intrusions such as Campi Flegrei, Italy west of Mt. Vesuvius, is limited to remote observations through geophysical/geodetic procedures. While the surface displacement due to melt emplacement at depth can easily be determined, the geometries and depth of intrusions are often based on simplified assumptions (e.g. spheres and prolate or oblate ellipsoids). These models benefit from data constraining both the geometries of the individual intrusions, and the kinematics and mechanics of deformation within the superstructure overlying the intrusions. Mount Ellsworth, a partially exposed sub-volcanic system, is an ideal natural laboratory for the study of near surface intrusions. The intrusions of the Henry Mountains are ideal because they were emplaced into relatively flat-lying stratigraphy of the Colorado Plateau, at a time when the stress field was largely isotropic. Previous geologic work done in the Henry Mountains, conducted by C.B. Hunt (1953) and Marie Jackson and Dave Pollard (1988), presents competing emplacement models (i.e. large batch intrusion or incremental sill growth), as well as, differing geologic map data and interpretations. Through a combination of 1:5000 scale field mapping and profile-oriented gravity study, we have produced detailed geologic maps and cross sections of Mt. Ellsworth assess the previous work done on Mt. Ellsworth with new datasets, as well as, evaluate criteria refining various emplacement models. Mapping results demonstrate that several of the assumptions made in models theorized by Hunt (1953) and Jackson and Pollard (1988), were inappropriately applied on Mt. Ellsworth. These assumptions include the thickness and separation of stratigraphic units, the size and distribution of sills and smaller intrusions, structural attitudes of beds and sills, and the presence of exposure of the main body of the intrusion. Gravity data collected on similar intrusions presented in Corry (1988) demonstrates the difficulty of obtaining a gravity anomaly on the wavelength of the assumed size of the intrusion. Forward gravity modeling of various potential geometries beneath Mount Ellsworth suggests that the anomalies are similar in shape with a magnitude between 16 and 20 mGal. Results from the gravity profiles collected for this study fail to predict an anomaly on the wavelength of the Mount Ellsworth intrusion and record a much more complicated anomaly than is presented by the forward models. By combining the stratigraphic data, structural data, and cross sections, it can be determined that the Mount Ellsworth intrusion is a laccolith with a floor 1.5 kilometers beneath the topographic surface, is 1 kilometer thick at its maximum, and has dimensions of 4 kilometers wide by 6 kilometers long.

Scholar Commons Citation

Nushart, Nathan, "Modeling Intrusive Geometries of a Shallow Crustal Intrusion: New Evidence From Mount Ellsworth, Utah" (2015). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/5753