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

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Abstract
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: F2 folds tighten, S2 cleavage becomes more penetrative, and L2 fold axis and intersection lineations show progressive change from southeasterly trending to northeasterly trending. L2 stretching lineations have a constant northeast trend. Rotation of second generation structures occurred by progressive noncoaxial deformation, with a northeasterly shear direction. Analysis of L2 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 F2 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.