PRIMARY AND TECTONIC FABRIC INTENSITIES IN MUDROCKS

In this paper, we present a summary of quantitative measurements of th e preferred orientation of phyllosilicate grains in a variety of tecto nically weakly deformed to undeformed, fine-grained, pelitic rocks. We also briefly examine a small amount of data on clay fabrics in deep s ea cores, and compare them to fabrics in deformed equivalents from the Mariposa Formation and Calaveras Complex, Sierra Nevada, California. The measured fabric ellipsoids support the following conclusions: (1) a bedding-parallel foliation forms in pelitic rocks during compaction that in our samples is associated with shortening between 3% and 74%; (2) the amount of compaction is in part controlled by the quartz-felds par content: interbedded sandy layers undergo little to no compaction and do not develop bedding parallel foliations; and (3) it is impossib le to determine precisely the amounts of compaction, horizontal shorte ning, and later tectonic strain in deformed samples because of the var iability of initial compactions. Our data, and results presented by ot hers, indicate that strain paths in pelitic rocks always include early compaction, followed by an episode of simultaneous compaction and pla ne strain in accretionary wedges, and by a variety of other types of s train in other settings.