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1 Department of Geology, University of Auckland, Auckland, New Zealand
The Kawhia synclinorium along the western margin of North Island, New Zealand, is a relatively simple open structure with a subvertical to steeply east-dipping axial surface and contains a Triassic-Jurassic shelf or arc-trench gap sequence. It is bounded to the east by a narrow zone of serpentinite and ultramafic rock corresponding to the magnetic Junction anomaly.
In the Waipapa and Torlesse terranes east of the Junction anomaly, three deformation phases can be recognized: (1) formation of mélange and imbrication of strata, with fold axes trending across the now-dominant basement grain and fold vergence predominantly toward the south; (2) strongly asymmetric folding and imbrication and further mélange formation on horizontal axes parallel to the present structural grain; folds verge to the east, and beds in the axial ranges have rotated to vertical and overturned attitudes; and (3) open folding on steeply plunging axes. Phases 1 and 2 are part of the Early Cretaceous Rangitata orogeny or predate it. There is no evidence yet for the age of phase 3 structures.
It is hypothesized that the rocks of the Waipapa and Torlesse terranes were imbricated and accreted in a suture zone east of the Junction anomaly. Phase 1 structures were formed oblique to the trend of the New Zealand geosyncline because of a strike-slip component of movement transmitted to the sedimentary column only before and during early décollement. Age patterns in the Torlesse and Waipapa terranes indicate that simultaneously with accretion the clastic apron prograded from south to north.
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