Abstract

Detailed field and petrofabric studies of the Precambrian gneisses in the Bear Mountain area of the Hudson Highlands metamorphic complex indicate two distinct episodes and styles of folding. The first produced isoclinal folds whose original orientations were modified by subsequent folding. This early deformation and concomitant metamorphism produced the well-developed regional foliation and much of the compositional layering characteristic of Highlands gneisses.

The second phase produced more open folds of a larger scale with axes plunging 30°-45° toward N. 35°-45° E. This superposed folding is responsible for the dominant northeast trend of Highlands structures. Hornblende and sillimanite c-axes parallel axes of both fold systems, and orientations of quartz (0001) and poles to biotite (001) also reflect both periods of movement. This suggests that both events occurred under similar metamorphic conditions and probably during a single episode of metamorphism.

Though other parts of the Highlands have not yet been studied in detail, structural data of other workers suggest a similar history elsewhere.

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