Presentation Title: The Spanish Creek mylonite: A newly recognized zone of high strain in the Northern Madison Range, southwestern Montana

Section: Poster Session Group 2 2

Location: , Great Hall

Date & Time: Friday, April 19, 2013 - 2:45pm - 3:30pm


Recent fieldwork in Archean rocks of the Northern Madison Range in southwestern Montana has revealed a zone of high strain approximately2 km long and at least 800 meters wide, herein referred to as the Spanish Creek mylonite.  A multipronged structural approach incorporating fieldwork, microstructural analysis, and geochronology is applied to study of this mylonite.  The dominant rock type in this area is granitic to granodioritic orthogneiss with variable porphyroclastic character.  The mylonitic foliation within the zone is dominantly NE-striking and NW-dipping (~225°/45°) and is often accompanied by a subhorizontal mineral lineation.  Examination of thin sections reveals the presence of high-temperature microstructures such as quartz ribbons and polygonal, recrystallized feldspar grains.  Microstructural differences between samples confirm field observations of strong strain gradients associated with this zone.  Electron backscatter diffraction results indicate a strong quartz CPO in all samples.  Based on this data, diffusion creep deformation mechanisms were likely dominant.      

Type of Presentation: Individual poster

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Presentation Topic: NA

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Presenter Information
Presenter(s): Johnson, Joshua E. (Josh)
Major(s): Geology
Class Year(s): 2013

Sponsor(s): West, David P. Jr.
Dept(s): Geology

Moderator: ,