Geology and evolution of the upper Nevis Basin

Boyce, Zara (Zara Lee)

The schist basement of the Upper Nevis Basin and Hector Mountains represents the Mesozoic collision between the Torlesse and Caples terranes. Three different schist lithologies (greenish-grey schist, dark greyschist, and unsegregated schist) are recognised within the Upper Nevis area. These represent the metamorphosed equivalents of quartzofeldspathic greywackes and argillites. The schists can also be divided into different textural zones, with unsegregated schists being TZ liB, greenish-grey schists TZ Ill, and dark greyschists TZ IV. The different schist groups also correspond to different terrane affinities, with unsegregated and greenish-grey schists being of Caples terrane affiliation, and dark greyschists Torlesse terrane affiliation. Mesozoic and Cenozoic structural features represent boundaries between the different schist types. Post-Miocene reverse movement on the Western Boundary Fault has folded the schist basement to the east into an antiform/synform pair, resulting in a remnant wedge of Manuherikia Group sediments lying on the basin floor. Uplift of the Hector Mountains has tilted the schist basement, while material eroding off the fault scarp has been deposited in the basin as Schoolhouse Conglomerate. The curre1_1t focus of tectonic activity has migrated basin ward to the Nevis Fault, where sediments and Quaternary terrace deposits are offset in a reverse sense. The antithetic Wrights Fault creates a mid-basin bulge. The southern end of the basin provides evidence of pre-Miocene normal faulting. Detrital garnets from the Miocene Manuherikia Group sediments and other reworked gravels show typical compositions of grossular35 almandine28 spessartine37 pyrope03• Such compositions are indicative of an Otago Schist source, north of the Upper Nevis Basin. Clast petrography of the Pliocene Schoolhouse Conglomerate shows that it was derived from a TZIV Torlesse affiliated schist basement, with no evidence for a TZ Ill Caples affiliated schist basement source. This suggests that it is derived from a source outside of the Upper Nevis Basin. Imbrication and sedimentary features indicate that the Schoolhouse Conglomerate was deposited proximally to the schist source, in a fluvial setting flowing from the north. Latest Quaternary terrace gravels contain TZ lli Caples affiliated clasts that are derived from the east and south. Hence the Upper Nevis Basin provides a good example of reversal of ancient transportation systems.

viii, 158 leaves : col. ill., maps (some col. 2 folded) ; 30 cm.

2002Boyce

POLYGON ((168.911691358357331 -45.41501624395066,168.782716154613126 -45.410021166257273,168.816688905933262 -45.264798367547776,168.936466515103092 -45.27002326296779,168.911691358357331 -45.41501624395066))