Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

The multiply deformed metsedimentary Devonian Littleton Formation in the Presidential Range, N. H., shows dramatic partitioning of Acadian D₁ strain at regional and outcrop scales. In the NW portion of the range with the highest elevations, D₁ is characterized by F₁ macroscopic folds and few mesoscopic folds. In the SE part of the range, at lower elevations, no macroscopic F₁ folds exist and mesoscopic folds dominate. The boundary between the NW and SE structural domains is remarkably well delineated by both the termination of macroscopic F₁ axial traces and the reorientation of F₄ hinge lines that refold the D₁ structures. This abrupt transition marking a change from lower (NW domain) to higher (SE domain) ductility is related to regional scale vertical crustal variations in deformation in the Acadian orogen.

L₁ mineral lineations composed of pseudomorphed andalusites are common. L₁ is sub-parallel to F₁ hinge lines and never seen in the principal stretching direction, perpendicular to the hinge lines. Reverse refraction of S₁ schistosity in andalusite bearing schists suggests that andalusite growth occurred prior to the main phase of D₁ deformation. Outcrop scale stratigraphic/rheological variations control whether L₁ develops. In thick (> 10 cm) massive schists, peudoandalusites are unaligned within the S₁ schistosity, whereas lithologies where original sedimentary bedding, S₀, is characterized by thin beds (< 10 cm) of alternating schist and quartzite, the L₁ mineral lineation is well defined. L₁ and F₁ hinge lines vary in orientation along the length of the D₁ macroscopic axial traces reflecting the non--cylindrical, sheath-like fold geometry for D₁ nappes. As such, the concentration of L1 lineations, which trend shallowly to moderately WSW, record the ENE transport direction of the easterly facing nappes.