The Caples/Aspiring terrane boundary—the translation surface of an early nappe structure in the Otago Schist

At the northern end of the Remarkables mountain range, Central Otago, New Zealand, psammitic schists of the Caples terrane pass downwards through a 300 m thick transitional zone into pelitic schists of the Aspiring terrane. Second generation mesoscopic structures vary in orientation and morphology with proximity to this shallow, southerly dipping terrane boundary: F 2 folds tighten, S 2 cleavage becomes more penetrative, and L 2 fold axis and intersection lineations show progressive change from southeasterly trending to northeasterly trending. L 2 stretching lineations have a constant northeast trend. Rotation of second generation structures occurred by progressive noncoaxial deformation, with a northeasterly shear direction. Analysis of L 2 lineations indicates a major strain gradient over the Remarkables, with maximum shear strains (> 100) immediately above the terrane boundary. The Caples terrane was thrust upon Aspiring terrane during ductile greenschist facies conditions of metamorphism by translation of a major F 2 nappe fold, the Remarkables Nappe, in a northeasterly direction. The transitional terrane boundary zone was the zone of high strain developed at the base of the Remarkables Nappe. Integration of calculated shear strains suggests a minimum horizontal displacement of 2 km and a maximum displacement in the order of 150 km.