Deformation partitioning during transpression in response to Early Devonian oblique convergence,northern Appalachian orogen,USA |
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| Institution: | 1. School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, Hampshire, United Kingdom;2. Applied Isotope Research Group, Departamento de Geologia, Universidade Federal de Ouro Preto, Ouro Preto, MG 35400000, Brazil;1. University of Arizona, Department of Geosciences, 1040 4th St., Tucson, AZ 85721, USA;2. Carnegie Institution for Science, Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, 20015-1305 Washington, DC, USA;3. Observatorio San Calixto, Calle Indaburo 944, Casilla, 12656 La Paz, Bolivia;4. Instituto Geofísico del Perú, Calle Badajoz #169, Mayorazgo IV Etapa, Ate Vitarte, Lima, Peru |
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| Abstract: | Transpressive deformation was distributed heterogeneously within the Central 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 folds, S>L fabrics and sub-parallel form lines, and asymmetric structures that together indicate dextral–SE-side-up kinematics. In contrast, intervening zones between ‘straight’ belts are characterized by open folds and L?S fabrics. Within both types of zone, metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade, including a penetrative, moderately to steeply NE-plunging mineral lineation. Thus, we interpret accumulation of plastic deformation and regional metamorphic (re-) crystallization to have been synchronous across the Central Maine belt shear zone system. Discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in ‘straight’ belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts. Kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the Central Maine belt shear zone system. Perturbations in the flow were caused by serially developed thrust-ramp anticlines in the stratigraphic succession immediately above the Avalon-like basement, at which décollement of the shear zone system was initially rooted. General shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow. In contrast, deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow. Part of the thickening stratigraphic succession exceeded Tsolidus, reflected by the occurrence of migmatites and granites. The latter were partly sourced from the underlying Avalon-like basement that was involved in the deformation and melting. |
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