CHARACTERIZATION OF MIDDLE PROTEROZOIC STRAIN IN THE MID-CONTINENT: EVIDENCE FROM THE “BARABOO INTERVAL” QUARTZITES (1700 MA) USING THREE STRAIN-GAUGE TECHNIQUES

Preliminary strain analysis of the “Baraboo interval” quartzites deposited throughout the Upper Midwest between 1750-1630 Ma was conducted using three independent strain-gauge techniques (Ramsey Method 3, Fry Technique, and the auto-correlation function [ACF]). Samples from the Baraboo, McCaslin, Flambeau, and Barron Quartzites of Wisconsin, yield strain-ellipses with long-axes that parallel regional fold axes. These data indicate approximately north-directed shortening, with a strain-gradient that decreases to the north, consistent with a post-Penokean thrust margin located south of present-day Wisconsin. These analyses provide evidence for Early-Mid Proterozoic deformation in Wisconsin, contemporaneous with the Mazatzal-Central Plains (southwest) and Labrador (northeast) orogenies that partially define the southern margin of the North American craton at 1650 Ma. In order to better characterize the orientation and intensity of the preserved strain regionally, additional sampling from northern Wisconsin localities and the Sioux Quartzite of Minnesota and South Dakota are in progress. Strain-ellipse equations from ACF measurements on three mutually-perpendicular sections from each sample will be combined in order to construct strain-ellipsoids, which will help to characterize the regional strain history. The ACF technique is an extremely efficient way to describe the correlation of shapes within rocks (in this case quartz grains) and has been used to quantify fabric intensities. The technique was found to yield strain-measurement results consistent with other established methods, and is therefore extremely promising as a strain-gauge technique.