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Paleozoic-Mesozoic Geology of South Island, New Zealand: Subduction-related Processes Adjacent to SE Gondwana

Robertson New ZealandNew Zealand is a very small part (about 6%) of a mostly-submerged continent, Zealandia. The emphasis of this volume is on the Paleozoic-Mesozoic subduction that occurred along the active SE Gondwana margin (SE Australia and possibly eastern Antarctica), leading to Zealandia’s rifting and separation from Gondwana in the mid-Cretaceous.

The concept of tectono-stratigraphic terranes is central to this study, and in South Island the terranes fall into two groups: those forming the Western Province (essentially intact Gondwana crust), and the somewhat larger Eastern Province (allochthonous units). By repositioning the terranes prior to the c.460 km Cenozoic dextral shift on the Alpine Fault, the Eastern Province terranes around Nelson at the north end of the island become contiguous with those of the Otago and Southland area at the other end. Recognizing this displacement was an important prerequisite to understanding how the terranes fitted into the picture of subduction along the Gondwana active margin.

The mountainous backbones of both the South and North islands are colloquially called ‘greywacke ranges’ (and the term greywacke has even been adopted as a brand of Marlborough white wine); this volume tells us that those mountains comprise rocks of the Torlesse Composite Terrane that formed as an accretionary wedge above the subducting Panthalassa oceanic slab. Most readers will have heard of the olivine-rich rock dunite; it takes its name from the Dun Mountain ophiolite, which we learn is a slice of  Panthalassa Ocean crust that forms part of the Dun Mountain-Maitai Terrane, pushed on to the Gondwana margin in the Permian.

The general reader will wish to read Chapters 1, 2, 3 and 15, which give an excellent overview of the island’s origins, while Chapters 4 to 14 provide more detailed evidence for where terranes of the Eastern Province, in particular, may have related to the active Gondwana margin. The book is well-written and lavishly illustrated, with contributions from an international team of experts. Many of the figures are in colour, but one small quibble is that the use of colour alone to distinguish units on a geological map can lead to difficulty for the reader; Figure 1.6 will be referred to repeatedly by readers, and yet it has no fewer than four shades of pink. The addition of symbols would have helped.  Nevertheless, I wish I’d had this book when I led a group of GA members around New Zealand a few years ago.

Reviewed by Michael F Ridd

PALEOZOIC-MESOZOIC GEOLOGY OF SOUTH ISLAND, NEW ZEALAND: SUBDUCTION-RELATED PROCESSES ADJACENT TO SE GONDWANA
, by Alastair H. F. Robertson (Ed) 2019, Geological Society of London Memoir 49, 378 pp. (hbk.) List price: £ 140.00 Fellow's price: £ 70.00 W: https://www.geolsoc.org.uk/M0049