The Main Divide Fault Zone and its role in formation of the Southern Alps, New Zealand

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Abstract
The Main Divide Fault Zone of the Southern Alps is a major fault system extending for a minimum of 60 km immediately below and east of the Main Divide. Regionally it strikes parallel to the Alpine Fault, but in detail is segmented with N‐NNE‐striking oblique‐reverse faults dipping 40–60° northwest, linked by steeper NE‐E‐striking, oblique strike‐slip structures. Dextral steps in the Main Divide follow segmentation of the adjacent faults, with major saddles above the NE‐E fault segments. The hangingwall rocks are relatively homoclinal, dipping c. 40° WNW, and composed of pumpellyite‐actinolite facies greywackes and semi‐schists with bedding transposed by anastomosing faults. The footwall rocks are less deformed, mostly non‐schistose prehnite‐pumpellyite facies greywackes and argillites, striking generally northeast (dip 50–85° northwest), but are folded by large kilometre‐scale, steeply plunging folds. Thermochronological data indicate significant vertical offset during the late Cenozoic. The Main Divide Fault Zone is a backthrust off the Alpine Fault plate boundary, and is fundamental to the uplift and strain within the Southern Alps.