Abstract

The Mount Desert Island granite is cut by fractures displaying one of four types of surface morphology: (1) smooth to undulatory; (2) stepped in the form of en echelon cracks; (3) striated with linear fibers; and (4) irregular with cataclastic grains. These surfaces belong to joints, host fractures with pinnate joints, reactivated joints or fractures, and deformation bands (shear fractures), respectively. Pinnate joints, like striations on slickensides, are structures indicative of the orientation of the slip vector and sense of shear on host fractures. Although fractures in the Mount Desert Island granite cluster into two major sets (N20°W and N45°E), host fractures with pinnate joints and shear fractures favor the N45°E orientation. A kinematic analysis of the pinnate joints indicates a predominantly dextral strike-slip sense of movement on northeast-trending fractures. This result agrees with previous work suggesting that a prominent postintrusion tectonic event in southeast Maine consisted of dextral strike-slip motion on northeast-trending faults.

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