CONNECTING PIECES OF THE ACADIAN PUZZLE IN THE NORTHERN APPALACHIANS

To understand the tectonics of orogens, we require knowledge of how the parts fit together and how deformation and thermal structure evolve. Taking a larger-scale view allows investigation of movement of rheological transitions [brittle-viscous transition zone (BVTZ) and anatectic front (AF)] as geotherms evolve with progressive deformation and erosion, and as melt advects heat to shallower levels. In the Acadian Orogen, Lower Paleozoic rocks were deformed synchronously with greenschist-granulite facies, low dP/dT type metamorphism and emplacement of plutons. In western Maine, deformation involved non-coaxial non-plane strain flow in which dextral-oblique contraction was partitioned into steeply inclined zones that record contrasting S-L and L>>S strains. Leucosome relations with tectonite fabrics and dilatant structures, and composite sheets and cylinders of cumulate-dominated granite suggest that feedback relations between deformation, melting and evolution of melt pressure cause spatially and temporally variable transient weakening leading to episodic melt escape. Thus, during prograde evolution, as the orogen heats and weakens the strain rate increases causing the BVTZ and AF to move shallower. Weakening localizes deformation in the orogen, where it facilitates melt transfer. On the pro-side of the orogen, in northern New Hampshire, E-verging km-scale recumbent structures are associated with emplacement of melt at the metamorphic peak. On the retro-side, in western New Hampshire and eastern Vermont, structural relations and counterclockwise P-T paths that imply loading at the thermal peak suggest moderately E-dipping thrust sheets translated W, each associated with syntectonic emplacement of melt at the apparent base. Thus, lateral expansion of the orogen at shallower crustal levels occurred by displacement along locally developed asymmetric detachments induced in the contact zone above bodies of melt trapped just below the BVTZ by developing recumbent structures. Finally, as heat input and rate of melt flow decline, residual melt trapped below the AF crystallizes and the orogen hardens, causing strain rate to decrease, rheological transitions to move deeper and deformation to partition away from the orogen and localize into the dextral-transcurrent Norumbega Fault Zone.