A review of the Median Tectonic Zone: Terrane boundaries and terrane amalgamation near the Median Tectonic Line

Abstract

The pre‐Late Cretaceous basement rocks of New Zealand are divided into a Western Province and an Eastern Province along a tectonic boundary called the Median Tectonic Line. The two provinces comprise assemblages of terranes of fundamentally different origin. The Western Province attained continental‐type thickness and structure by the end of the Carboniferous. The Eastern Province developed as a result of convergent margin crustal processes in the Permian‐Cretaceous. The boundary between the two is drawn in one of two positions in Nelson, either east or west of the Drumduan Terrane. The evidence presented so far on the affinities of the Drumduan Terrane is equivocal, and the suggested eastward extension of the Tuhua Orogen is not well supported. To the south, similar difficulties of interpretation occur, and it seems that between the Takaka Terrane in the west and the Brook Street Terrane in the east there is a belt of enigmatic rocks best termed the Median Tectonic Zone. In a broader context, the close proximity of the intra‐oceanic Brook Street Terrane and the continental Western Province suggest considerable telescoping or tectonic excision. The Drumduan Terrane, Largs Terrane, and other units represent small relics of a once more extensive intervening zone, in some places stitched to the Western Province by younger plutons. The rocks comprise the only available record of events along the Permian‐Mesozoic Gondwana margin.