Deformation partitioning during transpression in response to Early Devonian oblique convergence, northern Appalachian orogen, USA

Transpressive deformation was distributed heterogeneously within the Centra l Maine belt shear zone system, which formed in response to Early Devonian oblique convergence during the Acadian orogeny in the northern Appalachians . 'Straight' belts are characterized by tight fords, S > L fabrics and sub- parallel form lines, and asymmetric structures that together indicate dextr al-SE-side-up kinematics. In contrast, intervening zones between 'straight' belts are characterized by open folds and L >> S fabrics. Within both type s of zone, metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade, including a penetrative, moderately to steep ly NE-plunging mineral lineation. Thus, we interpret accumulation of plasti c deformation and regional metamorphic (re-) crystallization to have been s ynchronous across the Central Maine belt shear zone system. Discordance bet ween inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in 'straight' belt rocks records shortening b y tightening of folds and greater reorientation of matrix fabrics with resp ect to porphyroblasts. Kinematic partitioning of flow was responsible for t he contrasting states of finite deformation recorded in the Central Maine b elt shear zone system. Perturbations in the flow were caused by serially de veloped thrust-ramp anticlines in the stratigraphic succession immediately above the Avalon-like basement, at which decollement of the shear zone syst em was initially rooted. General sheer deformation at the ramps involved st rain softening with an enhanced component of noncoaxial flow. In contrast, deformation during extrusion in the intervening zones involved strain harde ning with a greater component of coaxial flow. Part of the thickening strat igraphic succession exceeded T-solidus, reflected by the occurrence of migm atites and granites. The latter were partly sourced from the underlying Ava lon-like basement that was involved in the deformation and melting. (C) 200 1 Elsevier Science Ltd. All rights reserved.