MULTI-DOMAIN MICROSTRUCTURAL FABRIC DEVELOPMENT AND MODIFICATION OF THE MORETOWN FORMATION IN CENTRAL VERMONT

The Moretown Formation in central Vermont contains a locally well-developed crenulation cleavage with alternating zones of muscovite + biotite + quartz +/- epidote limbs and alkali feldspar-dominated hinges. Quartz + alkali feldspar zones dominate isolated 1-3mm horizons that preserve structural heterogeneity not recorded by mica-dominated horizons. In the Elmore/Woodbury area of central Vermont, Taconian structures are preserved as north-trending, steeply-plunging folds with asymmetric limbs, shallow to the east. Well-defined, crystallographically preferred quartz and feldspar mineral lineations are warped, but discernable along NNE axial traces of Taconian fold structures. Shear bands (Acadian) offset shallow limbs with a dextral sense-of-motion and are associated with the axial trace orientation of the earliest generation of folding. Stretching lineations are superimposed on these (early) fold structures and appear to be associated with relatively symmetrical, shallowly north-plunging folds (Acadian). Taconian structures are increasingly attenuated from west to east across the area approaching the Richardson Memorial Contact (RMC). The RMC is similarly tectonized in this area, as has been described in locations in Vermont to the south, where earlier (Taconic) fold axes match later (Acadian) cleavages. Compositional mapping suggests that there are multiple generations of mineral growth that track the timing of later (Acadian) fabric development. Alkali-feldspar growth in low-pressure fold hinges appears to be synchronous with development of crenulation cleavage in mica-dominated domains. These domains are associated with the latest phase of regional metamorphism and deformation. Quartz + alkali feldspar-dominated horizons preserve significant shortening, but do not record subsequent shearing. This suggests that static grain-growth is much later than macroscopic shape-preferred orientations. The amount of mass transport needed to produce the youngest fabric can be correlated with the availability of readily accessible inherited structures. This may relate to a mechanism of fabric evolution different from that which is currently described for the eastern Moretown Formation in other localities.