Paleobotanical solution to a granite conundrum: Hawks Crag Breccia of New Zealand and the tectonic evolution of the southwest pacific

Abstract

The age of the Hawks Crag Breccia, previously thought to be Late Jurassic or older, is shown to be Late Albian by comparison of its spore‐pollen flora with dated Jurassic and Cretaceous assemblages in New Zealand and Australia. This age finally resolves the conflict between the presence of Cretaceous granite boulders in a supposedly Jurassic breccia. The revised age allows the tectonic evolution of the southwest Pacific to be more clearly resolved, into the following events: (1) development of volcanic arc above interface between converging Gondwana and ancient Pacific plates, with eugeosyncline and miogeosyncline on Pacific side; (2) start of Rangitata Orogeny in late Jurassic and earliest Cretaceous with emergence of eugeosyncline, transformation of outer “miogeosyncline” into eugeosyncline with new volcanic arc; (3) granitization and thrusting of Gondwana foreland; (4) deep synclinal folding of volcanic arc and eugeosyncline; (5) initial transaxial thrusting of outer geosyncline, followed by paraxial close folding; (6) swinging of all these structures from initial east‐west into north‐northeast and south‐southwest direction, with crowding of folds, and development of mylonite bands along stretched limb; (7) elevation of mountains, especially along stretched limb, and deposition of terrestrial formations such as Hawks Crag Breccia; (8) relaxation of stress, intrusion of mafic igneous rock, widespread gradual submergence spreading from Cretaceous geosyncline, closure of Cretaceous volcanic arc; (9) new break along recurved arc along transform “Alpine” Fault in middle‐late Tertiary.