OVERVIEW OF THE ARCHITECTURE AND DEVELOPMENT OF ZEALANDIA

Clark Blake was an enthusiastic participant in trans-Pacific exchanges and published half a dozen papers on the geology of New Caledonia and New Zealand in the 1970s. In the last 25 years, our understanding of the continental geology of the SW Pacific has been transformed by the Blake-Howell-Jones terrane concept, by the now-almost-routine U-Pb dating of zircons, by deep crustal geophysical profiles, and by the integration of offshore with onland geology.

Zealandia is a continent with an area about two thirds that of the conterminous United States, but which is 95% submerged. South Zealandia and Coastal-Baja California comprise the only continental portions of the Pacific Plate (about 2% of its area). The Mesozoic greywacke-dominated accretionary Torlesse Terrane with its complex structure and medium P/T “M1” Haast Schist metamorphism has drawn the most comparisons with Franciscan geology. However, the post-accretionary development of Zealandia and California, and reasons why these two pieces of continental crust came to be part of the Pacific Plate, are quite different.

Subduction ceased beneath the Torlesse Terrane at about 100 Ma, probably because of the collision of the Hikurangi Plateau Large Igneous Province with Gondwanaland. The total lack of preserved blueschist and eclogite metamorphic facies in the Torlesse may perhaps be attributed to core-complex style low P/T exhumational “M2” overprinting of the Haast Schist. Indeed much of the crust of what is now Zealandia and West Antarctica, especially under the Paleozoic-Mesozoic Median Batholith, underwent Late Cretaceous (100-85 Ma) tectonic thinning prior to final breakup of Gondwanaland and creation of Zealandia, Australia and Antarctica at 85 Ma. From this time the lodged Hikurangi Plateau probably played a major role in the retention of continental South Zealandia on the Pacific Plate and in the subsequent anchoring of the south end of the festoon of Cenozoic SW Pacific arcs and backarc basins.