GRENVILLE- AND ALLEGHANIAN-AGED DEFORMATION RECORDED IN THE GRANDFATHER MOUNTAIN FORMATION METACONGLOMERATE

The Grandfather Mountain Formation (GMF) contains metamorphosed shale, siltstone, sandstone, conglomerate, and both felsic and mafic volcanic rocks, and records the failed rifting of Rodinia (750 Ma). The GMF composes the majority of the Grandfather Mountain Window with exposures at varying distances from the Alleghanian-aged (Paleozoic) detachment fault underlying the Blue Ridge Thrust Sheet. The GMF is unique because it records Alleghanian deformation due to its position along the Linville Falls fault, but also contains clasts with relict Grenville age (Mesoproterozoic) deformation. We sought to distinguish between Alleghanian and inherited fabrics and determine the deformation and strain conditions, through microstructural and electron backscatter diffraction (EBSD) analysis of both matrix and clast quartz.

In thin section, both quartz clasts and matrix grains are characterized by fine, irregular and serrate grain boundaries, in addition to straighter and slightly coarser grained subgrains and new grains, and swaths of quartz crystals with similar extinction patterns. These microstructures are characteristic of bulging recrystallization and subgrain rotation recrystallization, both of which indicate greenschist facies conditions. In contrast, some clasts display larger scale serrate, irregular textures, suggesting that they may have experienced fast grain boundary migration at much higher amphibolite facies conditions. Quartz c- and a-axis pole figures made using EBSD show fairly random patterns for quartz clasts, and somewhat clearer, but still diffuse point maximums and girdles in matrix grains. Inverse pole figures for quartz clasts show strong evidence for prism <a> slip in all samples, whereas matrix quartz grains show an increased component of rhomb <a> slip, consistent with lower temperature deformation in these grains. Rf-phi analysis of both clasts and matrix grains yields relatively low strains of less than 2, with many samples having more highly strained matrix grains, suggesting strain has been partitioned into these smaller grains. Overall, the GMF conglomerate records higher temperature deformation in clasts derived from Grenville sources, which is strongly overprinted by pervasive greenschist facies deformation during Alleghanian thrusting.