MIGMATITES, GRANITES AND SHEAR ZONES: THE SIGNATURE OF APPALACHIAN SUTURES

The connection between migmatites and shear zones is fundamental to the tectonic history of the Appalachian orogenic core. Migmatites and granites are common at all scales in Appalachian shear zones (e.g., the Norumbega and Central Maine belt shear zone systems in ME and NH), and it is clear that during orogenesis of the N Appalachians, the process of generation, segregation, ascent and emplacement of granite magma is important to the deformation and metamorphic history. Indeed, migmatite and granite domains are present in and around shear zones located at each terrane boundary in the metamorphic core. The migmatites and granites have fabrics and geometries at the cm (leucosomes) and km (plutons) scales that define the orogenic structures. This is likely to have occurred because melt movement has important affects on the thermal and rheological behavior of the crust. The mechanism by which melt is transferred is debated, however, there is a clear link between textural, compositional and geometrical observations in migmatitic terranes showing migmatization and granite body construction to be part of the deformation within the orogen. Within migmatites, the geometry of leucosomes and granites may record the melt flow network through the crust that occurred during shearing of the melt-bearing rocks. This is supported by experiment, but also by observations of migmatites with granite in structurally-controlled sites. The implication is that in a partially molten rock, melt is segregated during deformation by moving down gradients in melt pressure to create leucosomes that form geometries controlled by the bulk strain ellipsoid. If deformation and melting are coeval, and as deformation progresses, strain is localized by changing rheology that results from reaction-enhanced ductility and melting. The introduction of melt into shear zone system is likely to be an effective weakening mechanism that will concentrate strain, suggesting a feedback mechanism. Does the presence of migmatite domains at tectonic terrane boundaries suggest that migmatites and granites stitch terranes together? Maybe not, but they apparently indicate the location of the fundamental deformation zones that are responsible for docking terranes.