FINITE STRAIN RECORDED BY GRAPTOLITES IN TACONIC SLATES
KIMBALL, Kari, and GOLDSTEIN, Arthur, Dept. of Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346.

kkimball@center.colgate.edu and agoldstein@center.colgate.edu

Graptolites present one of the very best potential indicators of finite strain. In particular, three species are abundant in slates of the Taconic folds and thrust belt and have been used for strain analysis. Climacograptus bicornis, Orthograptus calcaratus and orthograptus Whitfieldi all have uniform thecal spacings and thecal aperatures which are perpendicular to the stipe. Previous workers (Goldstein and Knight, 1996) measured graptolites at 6 localities. This study adds four additional sites to the analysis. Two sites collected for this study lie on the limbs of isoclinal folds with bedding and cleavage parallel. The additional two sites lie intermediate between hinge and limb of folds with bedding-cleavage angles of 25 and 41 degrees. The two cleavage-parallel sites return values (1+e1) of 1.29 and 1.37 for "X" and 0.80 and 1.04 for "Y", similar to the average of 1.65 and 0.85 for the other four limb sites. The two intermediate sites return maximum and minimum strains which are not principal and have values of 1.02 and 1.06 (max) and 0.39 and 0.45 (min). The minimum values can be used to help constrain the value of "Z" and indicate that it must be considerably less than the lowest indicated strain, in harmony with previously determined "Z" values of .40 and .20. Thus, the additional four sites corroborate previous results. Combining the four additional sites and using the average strains to determine volume loss leads to the conclusion that, on average, 59% of the original volume of the rock was removed. It is only at the statistical limit of our measurements that little or volume reduction is indicated. Some of the lost volume can be accounted for by collapse of smectite to illite, but the majority of the indicated volume loss must be attributed to pressure dissolution of quartz, calcite and chlorite. These results are in harmony with geochemically derived estimates of volume change (Brauer and Goldstein, 1994, Goldstein and Brauer, 1995). A comparison between graptolite and other strain measurement techniques applied in the Taconics shows that graptolite strains are quite similar to strains derived from measurements of reduction spots, once one accounts for burial compaction not recorded by graptolites. Further, graptolites consistently record lower elongations in "X" than do either pyrite framboid or pyrite porphyroblast pressure shadows, suggesting that they may not be useful as indicators of strain magnitude. Amounts of shortening recorded by buckled beds and veins are quite close to the "Z" values recorded by graptolites.

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