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 strike‐slip 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.