THREE-DIMENSIONAL STRAIN ANALYSIS OF QUARTZ ARENITES FROM THE ANTIETAM FORMATION, WESTERN BLUE RIDGE, VIRGINIA

Quartz arenites of early Cambrian Antietam Formation are penetratively deformed on the western limb of the Blue Ridge anticlinorium in north-central Virginia. The purpose of this study is to quantify three-dimensional strain in these quartz arenites across a series of gently plunging NW-verging overturned folds to determine if microscale strain is associated with folding during the Alleghanian Orogeny. The Antietam Formation is composed of medium-grained, well-sorted quartz arenites with abundant Skolithos. Undulose extinction, concave grain boundaries and discontinuous selvages of Fe-oxides form the prevailing microstructures. In highly strained samples deformation lamellae are common, whereas subgrain recrystallization is minor. The microstructures indicate that intracrystalline plasticity and pressure solution were the predominate deformation mechanisms in the Antietam Formation and developed under the lowest greenschist facies (<350° C). Elliptical Skolithos tubes graphically illustrate the penetrative nature of strain in these rocks, but are limited as they are two-dimensional strain markers. Finite strain analysis was performed using both the R_f/Φ and normalized center-to-center methods on detrital quartz grains. Three sections were analyzed per sample and combined to convert two-dimensional strain ratios into three-dimensional strains. K values range from 0.2- 3.6, with a majority of samples plotting in the field of apparent flattening. Deformation values range from 0.2- 0.9. Samples in the northern part of the study reveal apparent volume losses. In the most deformed samples X/Z strain ratios exceed 3:1. Strain is generally greater on the overturned limbs of folds. However, the orientation of the maximum principal stretch (X) has no clear pattern with respect to folding, furthermore foliation throughout the study area is folded. These observations indicate the Antietam Formation experienced penetrative strain prior to folding.