Document Type

Honors Thesis

Publication Date

5-2012

Abstract

The Grenville province is the exhumed remains of a more than 1 billion year old orogen. Mid to lower crustal rocks of this orogen are now exposed at the surface, affording the opportunity to examine deeper crustal processes during orogeny. I have studied an outcrop of anorthositic gabbro from the Central Metasedimentary Belt boundary thrust zone (CMBbtz) in the Grenville province of southern Ontario. The CMBbtz is a region that separates two major lithotectonic domains of this part of the Grenville province and is thought to have accommodated large scale thrusting in the mesoproterozoic era. The CMBbtz comprises large crystalline thrust sheets in a marble tectonic breccia and many anastomosing ductile shear zones. My outcrop occurs in the newly proposed Salerno Creek deformation zone which is thought be a major shear zone of the CMBbtz.

I have examined an outcrop that exhibits meter-scale shear zones and ductile strain localization. I have documented textural, chemical and microstructural characteristics across the varying degrees of strain. Based on these observations, I have proposed mechanisms by which these rocks have become locally weak, allowing strain to localize. The two primary minerals in these rocks, hornblende and plagioclase, appear to have deformed primarily by brittle fracturing and subgrain rotation recrystallization, respectively. Possible mechanisms of shear zone localization include original heterogeneities in the outcrop, the introduction of fluids and stress concentration in cross-cutting veins. There are many complex factors controlling these latter two mechanisms.

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