Abstract

An andesite tuff of Ordovician age from the Lake District, England, contains accretionary (hailstone) lapilli. These originally spherical bodies allow a tentative analysis of their history of deformation, at least for the single, isolated point from which a sample was taken. The lapilli were first deformed into oblate spheroids by compaction which shortened them vertically 53 percent, then the bedding plane was tilted 21° to the south-southwest. Thereafter the rock was converted to a chlorite-muscovite slate, shortened 50 percent along a near-horizontal north-south line, lengthened 100 percent near vertically, and left unchanged along the east-west horizontal. The direction of greatest shortening is normal to the cleavage, that of greatest lengthening parallel to a lineation in the cleavage plane. An X-ray diffraction pole-figure from three mutually perpendicular sections scanned in transmission indicates a preferred orientation of 9 times random of the basal planes of chlorite parallel to the cleavage plane. Preferred orientation of muscovite is similar to that of chlorite. Passive rotation of originally random platelets during the tectonic strain could explain preferred orientation of 8 times random; preferential post-tectonic growth of platy minerals parallel to their base, more often stopped by impingement on other grains if grains form large angles with the cleavage, may have enhanced the orientation.

Folds with axial surfaces different from the plane of slaty cleavage were observed 350 m from the sample point. If one assumes, somewhat tenuously, that the strain leading to slaty cleavage was fairly homogeneous over this distance and was the last strain to affect the rocks, then the shape of the folds before the cleavage episode can be reconstructed. Such a reconstruction of one fold and of its pre-cleavage fold axis and axial plane gives an idea of the shape of this fold as it may have existed after the pre-Bala folding episode and before the final deformation sometime during the Devonian Period.

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