Aspects of Deformation and Strain in the Appalachian Fold -Thrust Belt (New York) and the Shear Zones of the Sveconorwegian Orogen (Norway)
This dissertation addresses aspects of structural geology in two locations. In the first chapter, detailed geologic mapping and preliminary cross-sectional construction define along-strike changes in the structural architecture of the Alleghanian (Late Paleozoic) northern Appalachian foreland fold-thrust belt, near the town of Rosendale in southeastern New York State. Results suggest that along-strike thinning (from south to north) of rigid units within the Silurian-Devonian stratigraphic sequence caused a decrease in the amplitude of megascopic folds and a decrease in the spacing of thrust faults. Changes were accommodated by development of lateral ramps and by the dying-out of faults into fault-related folds. The second chapter compares results of different techniques for measuring grain-scale deformation in samples collected from exposures of the Late Silurian Binnewater Sandstone near Rosendale. Normalized-Fry and Rf/&phis; strain-analysis methods suggest that this unit accumulated only very low shortening strains (e.g., Fry results yielded <3%; max. 8%). Notably, long axes of grain-scale fabrics revealed by Fry analysis do not consistently parallel regional structural grain defined by trends of fold axes and cleavage in the fold-thrust belt. Thus, these fabrics may be a composite of two strain increments---primary (due to burial) and tectonic (due to the Alleghanian orogeny). In contrast, analysis of the anisotropy of magnetic susceptibility (AMS) yields magnetic lineations that closely parallel structural trends, even at localities where Fry fabrics do not, suggesting that AMS is a more sensitive indicator of tectonic strain low-strain rock. The third chapter explores the use of AMS as a tool for kinematic analysis in samples of Precambrian mylonite from shear zones in the Sveconorwegian orogen of southeastern Norway. The goal of this work was to provide constraints on transport directions in a case where outcrop-scale mineral-stretching lineations are missing or ambiguous, and quartz does not display crystallographic preferred orientation (even when analyzed by SEM backscatter). The results of this analysis suggest that AMS fabric analysis may provide a promising alternative method for determining lineations, but that its use requires detailed knowledge of magnetic mineralogy.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign