Abstract:
The area studied in the Kaweka Range consists of mainly Torlesse rocks, with a localized covering of Upper Tertiary sediments.
The steeply dipping Torlesse rocks can be grouped into four lithological associations; bedded sandstone, alternating, thinly alternating, and argillite. Deposition from grain flows, turbidity currents, and traction currents in a submarine fan environment is envisaged.
The detrital material of the sandstones and fine conglomerates indicates mixed volcanic, sedimentary, and ? plutonic sources. A nearby terrain of previously indurated "Torlesse" rocks, penecontemporaneous erosion of the unindurated sediment, and an acid - intermediate volcanic source, with some active volcanism, is inferred.
The following sequence of deformation can be recognized : 1. Soft sediment and early deformation, 2. Isoclinal folding, 3. Shearing leading to the development of broken formation, 4.Tectonic sliding giving rise to east verging folds, and 5.Oversteepening of bedding. Up to this stage deformation took place during imbrication in an active subduction zone. Large steeply plunging sinistral folds were then produced in a regime of transcurrent movement towards the end of the Rangitata Orogeny.
The Tertiary sequence begins with very localized conglomerate and sandstone units deposited on an irregular greywacke erosion surface. These units are separated from a more wide spread conglomerate - sandstone sequence, by a mid - Pliocene tectonic break.
Following the deposition of Tertiary material, overthrusting and uplift in the area took place along northeast - southwest trending faults, with a moderate component of dextral strike-slip. These faults are prominent features today, some showing a certain amount of recent normal movement.