Lithostratigraphy and structure of the Caples terrane of the Humboldt Mountains, New Zealand

Abstract

Abstract Schistose lithostratigraphic units, correlated with non-schistose rocks of the Caples terrane, can be mapped successfully along the western margin of the Haast Schist terrane in the Humboldt Mountains of north-west Otago. The rocks include chert, volcanogenic sediment and lava of the Harris Saddle Formation; the mineralogically immature Momus Sandstone; the new Cosmos Formation, a thin but persistent horizon of green volcanogenic sandstone; and the overlying pelitic and psammitic rocks of the Upper Peak unit, the Mystery Pelite (new), and the Bold Peak unit. They have been reconstituted under metamorphic conditions of the pumpellyiteactinolite and greenschist facies. Three pe1iods of deformation are recognised. The first, synmetamorphic phase resulted in a maior recumbent fold, with an axial surface parallel to the regional schistosity; the second, post-metamorphic phase is defined by folded schistosity and deformed isotects. Major structures of both fold episodes have been verified by direct obse1vation. Penecontemporaneous faulting was an important adjunct to post-metamorphic folding. In a third deformational episode the Greenstone Mélange, a chaotic rock unit containing a variety of igneous, metamorphic, and sedimentary rocks, was emplaced, possibly diapirically, along a narrow extensional zone in the east. Rocks of the Caples terrane grade northwards and eastwards into the Haast Schist terrane which in turn grades eastwards into Torlesse rocks. Caples and Torlesse rocks are petrographically and lithologically distinct; the Haast Schist thus appears to represent a metamorphosed amalgamation of these two rock suites.