EXTENSIONAL TECTONICS OF THE ACADIAN OROGENY

Tectonic models for the Devonian Acadian orogeny in the New England Appalachians typically invoke two major periods of crustal shortening: an early phase of recumbent folding followed by an upright doming event. In the western part of the Acadian orogen, Devonian deformation overprinted the Ordovician Taconic thrust belt. An alternative interpretation of the numerous domes in the western part of the Acadian orogen is that they record large-scale ductile extension. Evidence for extension includes dramatic tectonic thinning of Paleozoic units and synmetamorphic exhumation of foot-wall rocks. The best evidence for normal faulting is in the Chester dome in southeastern Vermont. There stratigraphic separation diagrams clearly show that Cambrian and Ordovician units have been excised by faulting; locally Middle Proterozoic basement rocks are separated from Silurian units by less than 0.5 km! Acadian extension occurred during garnet-grade metamorphism on low-angle, nearly-planar normal faults. Kinematic indicators reveal that hanging wall rocks were displaced to the southwest. The Chester dome is defined by a folded mylonitic foliation. During dome-stage folding, the mylonitic foliation and the tops-to-the-southwest kinematic indicators, such as C-S fabrics and rotated porphyroclasts, were complexly overprinted by simple shear into different patterns on the east and west sides of the dome. Acadian extension was most likely the result of gravitational collapse following surface uplift and thermal weakening of the lower crust.