Structure and neotectonics on the Main Divide, upper Wilberforce valley, mid Canterbury, New Zealand

Abstract

The upper Wilberforce valley drains the Main Divide of the northern portion of the actively rising Southern Alps. The Wilberforce area lies in a transition zone between oblique convergence to the south and a predominantly strikeslip segment to the north. The structure of the area is dominated by the 2 km wide, currently active, Main Divide Fault Zone (MDFZ), which strikes northeast and consists of two main strands. The western strand crosses the Main Divide at Browning Pass and dies out northwards. The eastern strand runs parallel to the Main Divide immediately east of the divide in the upper Wilberforce valley and then crosses the divide farther north. A broad region of relatively low relief and topography at Browning Pass has developed between the two fault strands. The MDFZ was initially dominated by southeast-directed oblique reverse faulting. Neotectonic features in the Browning Pass area, between the two fault strands, show dextral strike-slip motion with as much as 50 m horizontal displacement affecting recently deglaciated surfaces but <10 m of vertical displacement.South of the MDFZ, the structure of the area is dominated by north-striking faults which intersect the MDFZ immediately east of the divide. These faults are subparallel to bedding in the host Torlesse Terrane sediments, and faults and bedding control topography. Bedding becomes rotated subparallel to the MDFZ along the Main Divide. A spaced cleavage in metasediments on the hanging wall of the MDFZ may indicate that the hanging-wall rocks are of higher textural grade than the textural zone I footwall rocks.

Upper crustal structure beneath the eastern Southern Alps and the Mackenzie Basin, New Zealand, derived from seismic reflection data

“…Field mapping indicates that considerable deformation and reorganisation of Torlesse greywackes occurred during Mesozoic as well as Cenozoic tectonism Spörli 1979;Ward & Spörli 1979;MacKinnon 1983;Findlay & Spörli 1984;Cox & Findlay 1995;Becker & Craw 2000). Earliest deformation probably occurred shortly after deposition (Permian to Late Triassic), in places before rocks were completely lithified, and was followed by pre-and synmetamorphic deformation events during Jurassic to Late Cretaceous before the late Cenozoic formation of the alps.…”

Gold mineralisation near the Main Divide, upper Wilberforce valley, Southern Alps, New Zealand

“…2). However, Wilsons Reward Reef host rock is considerably more sheared and disrupted due to its locality in the more intensely deformed part of the Main Divide Fault Zone (Becker & Craw 2000). Bedding is barely recognisable in many nearby outcrops, and is almost obscured at the macroscopic scale (Fig.…”

“…These veins are readily distinguished mineralogically and structurally from postmetamorphic Bell & Fraser (1906) (see Table 2). tension gashes and silicified zones (described below), which are associated with soft gouge zones and thrust faults of the Late Cenozoic Main Divide Fault Zone (Becker & Craw 2000). Rocks of the Torlesse Terrane in the upper Wilberforce valley are highly sheared and form substantial crush breccias (10-100 m) within a fault zone analogous to Cox & Findlay's (1995) (Becker & Craw 2000).…”

“…3A, B, C). Veins cut rock in the Main Divide Fault Zone in the upper Wilberforce valley, which was deformed by reverse faults in the late Cenozoic as the Southern Alps rose, and is currently being deformed by strike-slip faults (Becker & Craw 2000). Farther west, veins cut steeply dipping schistosity which was rotated from near-horizontality and exhumed in the late Cenozoic (Fig.…”

“…The southwestern end is little-deformed prehnitepumpellyite facies metagreywacke, passing into interlayered metagreywacke and argillite, which has been variably (Fig. 3A) (Becker & Craw 2000). Veins strike subparallel to bedding and locally cut across individual beds.…”