THE NORUMBEGA FAULT ZONE, MAINE - A MID-LEVEL TO SHALLOW-LEVEL CRUSTAL SECTION WITHIN A TRANSCURRENT SHEAR ZONE

From studies of structure, metamorphism, and geochronology we have evi dence that the Norumbega Fault Zone represents a transition from mid- to shallow crustal levels in a dextral, transcurrent shear zone within the northern Appalachian Orogen. The Norumbega Fault Zone strikes par allel to the orogen (northeast-southwest), is similar to 5 to 30 km wi de, and is characterized by distributed ductile dextral shear fabrics in the southwestern section with a transition to brittle fabrics towar d the northeast. Within the zone of distributed shear, deformation is partitioned into local zones of very high strain. Upright, isoclinal f olds are common in areas of high and low strain. Metamorphic grade dec reases from amphibolite facies in the southwest to sub-greenschist fac ies in the northeast. Ar-40/Ar-39 mineral ages from recrystallized min erals in a high strain zone, regional cooling ages in areas of lower s train, and metamorphic textures are consistent with a polyphase histor y of deformation. We interpret a younging trend in Ar-40/Ar-39 cooling ages toward the northeast, together with the deformational fabrics an d metamorphic features, to represent exhumation of the southwestern se ction of the Norumbega Fault Zone from mid-crustal levels during the p olyphase history of this transcurrent zone. The Norumbega Fault Zone m ay therefore serve as a model for deformational processes at mid- to s hallow crustal levels in active strike-slip systems.