Geometry and Progressive Development of a Shallow Crustal Intrusive Complex, Mount Hillers, Henry Mountains, Utah

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Robert Joseph Broda (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: The Henry Mountains of southeastern Utah are five topographic domes, each of which is a separate mid-Tertiary shallow crustal intrusive complex comprised of a cluster of component igneous intrusions. The intrusions were emplaced at shallow depths into flat-lying sedimentary strata of the Colorado Plateau. The lack of tectonic influence imposed on the intrusive bodies during or since emplacement makes the Henry Mountains an ideal place to study forceful magma emplacement in the shallow crust. This study focuses on the internal structure of the Mount Hillers intrusive complex which has an estimated igneous rock volume of approximately 35 km³. New geologic mapping demonstrates that the intrusive complex includes several component intrusions with distinct geometries. The largest component intrusion of the complex is a concordant, asymmetric laccolithic body with Permian sedimentary units contacting the southeastern roof and Jurassic sedimentary units contacting the northwestern roof. The different aged sedimentary units in contact with the upper portion of the main laccolithic body imply that the elevation of the base of the intrusion is not consistent within the stratigraphic section. The two areas with different-aged roof and floor units appear to be separated by a major but poorly exposed fault. Additionally, the main asymmetric laccolithic body appears to be directly attached to and likely once fed smaller laccolithic bodies on its northern perimeter. This improved understanding of the detailed internal structure of component laccolithic bodies of Mount Hillers provide constraints on the progressive developmental history of the intrusive complex. In the early stage of intrusive development, a network of sills and dikes was emplaced into subhorizontal strata. Next the main feeder dike of Mount Hillers (hypothesized to strike northeast) directly fed one southeast-radiating semicircular protolaccolith under Permian units. As the intrusion grew vertically, a fault may have developed parallel to and above the main feeder dike as the intrusion bent and uparched overlying host rock southeast of the fault. Once the intrusive body grew to a vertical thickness of ~1,500 meters, magma may have started to intrude laterally beneath Jurassic units to the northwest. The main laccolithic body then continued to grow vertically, simultaneously lifting different aged sedimentary roof units that are separated by a large fault. Late in the intrusive history, several relatively small tongue-shaped laccolithic intrusions were fed from and grew adjacent to the northern margin of the main asymmetric laccolithic body of Mount Hillers. The Henry Mountains are where geologist G.K. Gilbert first recognized and documented that magma bodies can deform host rock. Gilbert used the term `laccolite' (later 'laccolith') to describe plutonic bodies that make space for themselves in the shallow crust by lifting and uparching overburden. Since Gilbert's early work, the Henry Mountains region has been the focus of considerable research on the geometry and growth of igneous intrusions in the shallow crust. The new work presented here reveals that intrusions that were once thought to be a clusters of separate igneous bodies may instead be an amalgamation of component laccolithic intrusions that make up a large and geometrically complex intrusive feature.

Additional Information

Publication

Thesis

Language: English

Date: 2023

Subjects

Geology;Asymmetric;Emplacement;Intrusive complex;Magma pulses;Shallow crust

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