VARIATION IN QUARTZ ARENITE DEFORMATION MECHANISMS BETWEEN A ROOF SEQUENCE AND DUPLEXES

Microstructural abundances and histories in quartz arenite of the Lowe r Silurian Tuscarora Sandstone were used to determine the nature and r ole of microscale deformation in a cover sequence and underlying thrus t system, and to assess the degree to which the cover sequence accommo dated emplacement of the thrust system. Sandstone samples are located across the transition from the central to southern Appalachian forelan d thrust system where the thrusts change from blind to emergent southw ards and where southern deformation of early Alleghanian age has been previously shown to be overprinted by central deformation of Alleghani an age. Microstructures observed with transmitted light and cathodolum inescence microscopy indicate that grain-scale deformation occurred by dislocation flow, pressure solution, and microfracturing, with the la st being generally the most important. The sequence of deformation mec hanisms is the same for the cover sequence and the thrust system: pres sure solution during sedimentary compaction; dislocation flow during l ayer-parallel shortening; and localized microfracturing with limited p ressure solution near major thrust ramps and in steep fold limbs. A gr eater abundance of dislocation flow microstructures in the cover seque nce from layer-parallel shortening indicates grain-scale accommodation in the Tuscarora Sandstone of some shortening associated with emplace ment of the thrust system. The transition zone between the central and southern Appalachians contains the greatest occurrence of every micro structure which is consistent with the area having been affected by di achronous central and southern Alleghanian deformations.