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Fabrics and strains in quartzites of a metamorphic core complex, Raft River Mountains, Utah

By
Robert R. Compton
Robert R. Compton
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Published:
January 01, 1980

Solid-state flow during metamorphism was studied in a 300-km2 exposure where Elba Quartzite forms the upper part of an autochthon and is overlain by two major allochthonous sheets. The quartzite is very locally thrown into north-verging recumbent folds, the largest having a wavelength of 1 km. Fold axes and elongate metamorphic grains trend approximately east, and foliation is subhorizontal.

A vertical sequence of samples was collected from the large fold, and a second sequence was collected from unfolded quartzite 4 km away. Axes of strain ellipsoids measured in five quartzites from the unfolded sequence are close to 0.5:1.1:1.7 (assuming an original sphere of rádius one). Five samples from the fold have ellipsoid axes ranging from 0.4:1.1:2.5 at the top of the sequence to 0.2:1.4:4 near the base. Most quartz grains deformed plastically without recrystallizing and developed strong c-axis fabrics. The degree of orientation of both quartz c axes and muscovite plates increased with increasing strain. Most of the quartz fabrics have orthorhombic symmetry and cross-girdle patterns like fabrics produced experimentally by J. Tullis and computer-simulated by G. S. Lister and coworkers, for quartzites extended in plane strain.

The results thus indicate a gradual east-west extension and consequent flattening by the force of gravity. Perhaps the fold formed concurrently where material flowing laterally under a broad dome was arrested locally and thus forced to buckle. Dating of nearby granite bodies indicates that the deformation began in Eocene time and continued until Miocene time.

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Contents

GSA Memoirs

Cordilleran Metamorphic Core Complexes

Max D. Crittenden, Jr.
Max D. Crittenden, Jr.
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Peter J. Coney
Peter J. Coney
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George H. Davis
George H. Davis
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Geological Society of America
Volume
153
ISBN print:
9780813711539
Publication date:
January 01, 1980

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