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Early deformation in melange terranes of the Ghost Rocks Formation, Kodiak Islands, Alaska

By
Tim Byrne
Tim Byrne
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Published:
January 01, 1984

The Ghost Rocks Formation represents the youngest phase of a Late Cretaceous to early Tertiary episode of accretion during which about 80 percent of Kodiak Island was added to the continental margin. The formation consists of both structurally coherent terranes and terranes of complexly deformed sandstone-shale melange. Two mesoscopically ductile and regionally occurring deformations and three younger, relatively brittle deformations have affected the melange. D1 is characterized by a foliation, S1, which dips northwest and a lineation, L1, which plunges gently southwest. S1 is defined by disrupted lithologic layering, pinch and swell structures, the alignment of inclusions, and an anastomosing cleavage in the shales. L1 is defined by pinch and swell axes, the long axes of inclusions, mullions, and folds of phyllosilicates in the shales. D2 is characterized by a foliation, S2, and a lineation, L2, which respectively dip and plunge parallel to the D1 fabrics. S2 is a closely spaced, roughly planar, slaty-like cleavage that commonly transposes, but only locally folds S1. L2 is defined by D2 fold axes and the intersection of S1 and S2 which locally is a pencil cleavage. D1 significantly and substantially altered the original stratigraphy in the melange, and the distinctive disruptive style of the melange is probably a direct consequence of this deformation. The most important structural element that contributed to this deformation, and therefore to S1 and L1, was a three-dimensional web structure of cataclastic shear zones. This structural element occurs in essentially every sandstone in the melange and suggests, but does not prove, that D1 was technically induced. Other common D1 structural features are calcite-filled veins, swirls and folds of detrital or diagenetic phyllosilicates, and planar shear zones in the shales. Asymmetric D1 structures, especially disrupted tuff horizons, suggest D1 occurred during northwest directed layer-parallel shear and underthrusting, although other noncoaxial strain histories are also possible. Layer-parallel simple shear, and coaxial plane, constrictional, and flattening strains appear to be ruled out, however, because they do not explain the general, mesoscopic strains observed in the melange.

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Contents

GSA Special Papers

Melanges: Their Nature, Origin, and Significance

Loren A. Raymond
Loren A. Raymond
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Geological Society of America
Volume
198
ISBN print:
9780813721989
Publication date:
January 01, 1984

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