Abstract:

SUMMARY: Two existing and two new methods of strain analysis have been used on samples of accretionary lapilli tuff from the English Lake District. Results from each method are presented for comparison. Factors responsible for lapilli shape are itemized and discussed. Regional studies indicate that lapilli ellipsoids contain initial, compaction and tectonic components. The most reliable results indicate that compaction of 66% of original thickness normal to bedding during diagenesis was followed by tectonic strains due to folding and cleavage formation. The effects of folding on lapilli shape are thought to be small. Slaty cleavage was produced by strains close to plane strain, indicating a shortening of SO-70% normal to the cleavage plane. It is concluded that slaty cleavage does not form perpendicular to the maximum total shortening of the rock, but perpendicular to the short axis of a strain ellipsoid associated solely with cleavage formation and computation of three-dimensional progressive deformations.