Linking strain accumulation and fluid-rock interactions in a naturally deformed diamictite

Dyanna M. Czeck1, W. Adolph Yonkee2, Amelia Nachbor1, Kimberly R. Johnson1

1University of Wisconsin-Milwaukee, 2Weber State University


Detailed structural and geochemical analyses of a diamictite that was deformed during movement of the Willard Thrust reveal gradients in strain and mineralogy, providing a natural laboratory to study processes of reaction softening and hydrolytic weakening. By linking measurements of strain, chemistry, and quartz water contents with microstructural development, we hope to better understand the style and scope of fluid interaction during deformation of these rocks.


Located in northern Utah, Antelope Island and Fremont Island lie within the footwall and hanging wall, respectively, of the Willard Thrust Fault. The Willard was active during the Sevier Orogeny, and its thrust sheet carried a 10- to 15-km-thick sequence of rocks over 60 km eastward. Included in this rock sequence is a Neoproterozoic diamictite called the Perry Canyon (hanging wall name) and Mineral Fork (footwall name), which is the primary focus of our study.  Many of the gneissic clasts from this diamictite are derived from erosion of the Farmington Canyon Complex basement gneisses found in the footwall.


Deformed diamictites at Antelope Island show field, microstructural, and geochemical evidence for fluid influx during deformation.  The strain is predominantly plane strain or minor apparent flattening, with exposures being progressively more strained to the NW.  The order of clast competence based on strain data is: quartzite > red orthogneiss > green paragneiss. Chemistry and strain analyses indicate that rocks underwent only limited volume loss on Antelope Island (footwall), but significant volume loss on Freemont Island (hanging wall).  On Antelope Island, deformed clasts indicate Mg gain and Na loss compared to protolith, similar to basement shear zones.  However the hanging wall has different chemistry.  Therefore, fluid chemistries were likely to be different on the hanging wall and footwall.  We used the IR beam at the UW Synchrotron to create detailed water maps within quartz.  The amount of water increases with increasing strain and is preferentially located along microcracks and fluid inclusion trails


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