Abstract:
The Hooker and Copland Valleys, Central S. Alps, N.Z., provide sections through two different but adjacent structural domains within the Central S. Alps. The eastern domain (Mt Cook Range, east of Hooker Valley) is underlain by the non-schistose faulted and folded "Alpine greywacke" of the Torlesse Terrane. Three episodes of folding occur throughout the eastern domain: F1Mt Cook folds are prelithification folds; F2Mt Cook folds are ½ km scale, steeply-plunging, sinistral, intraformational isoclinal folds that, in east younging sequences, face south; F3Mt Cook are extremely large (wavelength 10 km, amplitude 8-20 km) regionally north-facing, dextral, steeply-plunging folds. The western domain (Copland Valley, between the Main Divide and Alpine Fault) is formed by the "Alpine schists" of the Haast Schist Terrane. Four fold episodes are recognized in the Alpine schists F 1Alpine schist folds are rare prelithification folds: F2Alpine schist folds are apparently recumbent, isoclinal folds, displaying an axial plane schistosity (S2), of m to ½ km scale; F3Alpine schist folds are regionally-developed, km scale, shallowly south west-plunging structures, formed of laterally and vertically anastamosing zones of transposition on a steeply northwest-dipping axial plane schistosity (S3) and which separate pods of less-intense F3 deformation within which is preserved the integrity of the earlier structures; F4 Alpine schist are mesoscopic upright folds sub-coaxially refolding mesoscopic F3Alpine schist folds, and occur only in the western part of Copland Valley. Eastern and western domains are separated by a 9 km-wide zone of faulting within the Main Divide. Principal fault-zones NE-trending trend NNE and NE and dip steeply west; the fault-zones strike from the Main Divide through the Mt Cook Range, and disrupt the F3Mt Cook folds. Three phases of faulting are recognized : pre-Miocene post-S2, pre-S3 possibly sinistral transcurrent slip on northnortheast trending faults; 38-10 my BP dextral transcurrent movement; 10 my-recent, syn- to post-S3 reverse faulting with transcurrent components. Azimuths of kinematic axis of shortening during the last two phases trend between 045º and 140º.Grade of metamorphism increases westward from prehnite-pumpellyite in the Mt Cook Range to possibly Sillimanite-muscovite Zone in the western part of the Alpine schists. Salient features of the multiphase metamorphism are : some prehnite veining in the Mt Cook Range predates microscopic parasitic F3Mt folds; other prehnite veins may be synchronous with post-F3Mt Cook reverse faulting; in the Alpine schists, a "static" period of porphyroblast growth intervened between a possibly Cretaceous, early, and a principally Miocene-to-Recent, late, phase of F3Alpine schist folding , and a dynamic recrystallization occurred during the late F3Alpine Schist phase. It is possible that high-temperature metamorphism occurred during the late-F3Alpine schist Tertiary deformation. Although the major structures in both domains are third generation folds, faulting in the Main Divide obscures the relationship between these geometrically very different structures. The f3Alpine schist structures are interpreted as initially buckles that, following their formation as a strike-slip fold system in a dextral wrench regime, were subsequently modified and transposed during a compressive deformation. During this later deformation, uplift of the Alpine schists occurred by quasi-plastic, aseismic, flow on the transposition zones now forming the steeply-dipping F3Alpine schist fold limbs. The F3Mt Cook folds could be recumbent folds that have been tilted and rotated to steep plunges on a large scale, or folds that formed as steeply-plunging dextral wrench folds after imbrication and tilting to steep dips. The deformation common to both domains is a rotation about a southwest-trending axis. This deformation produced, as "deep crustal buckles" the early phase of F3Alpine schist folds, and was represented in "near surface" regions by imbrication and rotation of bedding to steep dips. If the F3Mt Cook folds initially were recumbent, then they 3Mt Cook are older than the early-F3Alpine schist folds; if they formed as dextral wrench folds, they could well be contemporaneous with the early-F3Alpine schist folds. Regional relations of these folds are reviewed, and the recumbent fold model is favoured. A brief geotectonic model is presented.