GRAPTOLITES AS INCREMENTAL STRAIN MARKERS IN LOW-GRADE METAMORPHIC ROCKS

Graptolites are well known for their use as markers for the measurement of finite strain. This has been accomplished primarily through the analysis of thecal spacing, which returns information on the orientation, shape, and size of the finite strain ellipse in the bedding plane. Detailed observations of graptolites in black slate of the Mount Merino Formation in the northern Taconic Allochthon, New York, indicate that graptolites can be used also as incremental strain markers.

Transmitted and reflected light microscopy indicates that graptolites in the Mount Merino Formation are composed of graphitic blocks that are surrounded by fibrous white mica, chlorite, and/or quartz. The taphonomic pathway taken by the graptolites consisted mainly of carbonization of the original organic remains. This material subsequently underwent thermal maturation during low-grade metamorphism. Deformation during slaty cleavage formation resulted in fracturing of the graptolites to form the graphitic blocks and precipitation of the fibrous material within boudin necks between blocks and as strain fringes adjacent to blocks. The spacing and orientation of the fractures are similar to the spacing and orientation of fusellar rings and cortical bandages observed on the sicula and thecal walls of very well-preserved graptolites. This suggests that the position of the fractures is controlled to some extent by anisotropies inherited from the organic remains of the graptolites.

Incremental strain data can be obtained in three ways: (1) by comparison of strains determined from thecal spacing and those determined from reconstruction of the graphitic blocks; (2) from the geometry of fibers in strain fringes adjacent to the graphitic blocks; and (3) from the orientation of shortened, shortened then extended, and extended graptolites. Application of these approaches to the graptolites in the Mount Merino Formation shows that during Taconian orogenesis the strata underwent an early, pre-folding episode of layer-parallel shortening and slaty cleavage formation occurred during top-to-northwest, sub-simple shearing.