Date of Award
Honors College Thesis
Geography and Geology
Jeremy Deans, Ph.D
Geography and Geology
The Appalachian range was formed by three orogenies, the most recent being the Alleghanian beginning at ~330 Ma. A small pluton outcropping along Highway 280 near Alexander City, AL, called the 280 granodiorite, has been dated at ~335 Ma and intrudes the Devonian Elkahatchee Quartz Diorite (EQD). While little work has been done on the intrusion, it is significant because of its age: all deformation of the intrusion should be related to the Alleghanian orogeny, while most other bodies in the region exhibit significant overprinting due to previous successive orogenies. The pluton was mapped constraining the geographic extent, which is relatively small with an exposure of ~0.20 km2. Fractures, faults, and foliations were measured, with normal faults striking NW - SE and dipping NE; fractures with a W - NW strike and steep SW and NW dips; and foliations with a N - NE strike and moderate SE dip. Oriented samples were taken at several locations for petrographic and microstructural analysis. Petrographic assessment suggests the pluton is a muscovite granodiorite with a metamorphic overprint. Microstructural analysis reveals quartz deformed by grain boundary migration recrystallization, indicating amphibolite grade deformation. Shear sense indicators suggest predominantly dextral, reverse shear, with a few normal sense of shear fabrics. Field observations of brittle deformation indicate dextral, south-directed extension. In comparison, the nearby Wedowee Group has amphibolite grade dextral and extensional features. The 280 granodiorite has both reverse and normal amphibolite grade kinematic indicators, suggesting a record of high temperature contraction during the Alleghanian orogeny and below greenschist grade (faults) extension during orogenic collapse and formation of the Mid-Atlantic Ridge.
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Rosenbalm, Avery, "Structural synthesis and spatial distribution of a 335 Ma igneous suite from the Eastern Blue Ridge, Alabama Appalachians" (2018). Honors Theses. 552.