QUANTIFYING THE STRAIN RECORD IN NATURALLY DEFORMED CONGLOMERATE, SILURIAN RANGELEY FORMATION, WESTERN MAINE

In an attempt to quantify the strain record in rocks deformed during the Acadian orogeny (Devonian) in western Maine, we use two different types of meta-conglomerate found exposed in the same 500-meter-long exposure of the Rangeley Form. This outcrop is within a regional scale zone of apparent flattening strain. In this exposure, the Rangeley Form. (base of Silurian deposition, Central Maine belt) contains a penetrative matrix fabric (greenschist facies) sub-parallel to relict bedding that strikes generally NE-SW, and steeply to moderately SE-dipping. This part of the Rangeley Form. consists of texturally heterogeneous rocks that define two main contrasting types of matrix-supported meta-conglomerate that alternate as layers at the meter scale. One type is dominated by boulder-size clasts (5 to 10 cm in diameter; ranging in composition from meta-igneous to meta-sedimentary rock), whereas the other type is dominated by pebbles (1 to 2 cm in diameter; predominantly meta-sedimentary rock). Relative to the pebble meta-conglomerate, the boulder meta-conglomerate, by virtue of clast sizes and rock type variations, has a larger grain size variation, and therefore a larger inferred contrast in rock strength. We illustrate and quantify the shape and orientation of the strain ellipsoid using R_f-f strain analysis performed in the field and laboratory (using oriented thin sections) on elliptical clasts exposed on ten variously oriented 2-D surfaces. The study documents the 3-D strain ellipsoids for size, shape, and variation of axial orientation across cm- to meter-scales. At the meter-scale, the textural differences between the two types of meta-conglomerate recorded strain simultaneously, yet differently apparently according to the bulk rock strengths. The apparent flattening strain recorded by the matrix mineral texture was apparently recorded by the clasts in both meta-conglomerate types. Results show a strong alignment of the maximum principal axis of the clasts sub-parallel to the intersection of the foliation with each surface (used as the analysis reference line). The 3-D strain determined using these clasts shows that the strain ellipsoid has a sub-vertical maximum principal axis, sub-parallel to the dip of the foliation regardless of conglomerate type, to reflect the record of tectonic strain.