Physical Geography and Sciences


Kayla Kopinski



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Kayla Kopinski, Geology
Faculty Mentor: Professor Gary Solar, Earth Sciences

Rocks exposed in southern coastal Maine, between Portland and Freeport, were formed over 400 million years ago, and then metamorphosed and deformed from about 400 to 280 million years ago within a more than 15 km-wide zone of continental collisional deformation known as the Norumbega shear zone system (NSZS) during the Acadian through Alleghenian Orogenies (mountain building during continent-continent collisions). Evidence of the tectonic plate collisions in question exists at the grain scale, and can be studied and measured through the use of polarized transmitted-light microscopy (petrography) of thin sections. Thin sections were prepared from specimens carefully selected from a suite of rocks collected in the field in southern coastal Maine. They were then described and measured extensively, with the main focus of the study on porphyroblasts, or larger mineral growths resulting from metamorphism. As porphyroblasts form, they overgrow the existing matrix fabric patterns, preserving parts of it as inclusions. While subsequent deformational events may alter the matrix minerals, these inclusion minerals are preserved and reflect the original matrix at the time of growth. Measurements were made of overall porphyroblast size, core and rim sizes, tail dimensions and attitudes, and inclusion trail geometry as it relates to the existing matrix fabric. All of these data are reconciled to form interpretations of the tectonic history as recorded by the rock itself. The data documented shows the extensive tectonic history within the Norumbega shear zone was long-lived as new matrix fabrics formed over old ones progressively.

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Microstructural Documentation of Metamorphic Rocks from Coastal Maine
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