The Mesozoic of New Zealand: Chapters in the history of the Circum-Pacific Mobile Belt

The Mesozoic includes the later history of the New Zealand Geosyncline, the Rangitata Orogeny that ended the geosynclinal phase, superposing a new structural system, and the beginning of the transgression that followed. The main elements of the geosyncline, from west to east, are: (1) a foreland, metamorphosed and intruded by granite in a relatively high temperature/low pressure environment; (2) a geosynclinal margin or hinge-line, the median tectonic line of Landis and Coombs; (3) a western or marginal belt of geosynclinal sediments (Hokonui Facies) relatively fossiliferous, apparently deposited over the Pacific edge of the sial and now forming a western marginal syncline; (4) an abrupt facies junction marked by ultramafic intrusions and Permian volcanics, perhaps the early Mesozoic oceanic boundary of sialic crust; and (5) an eastern belt of relatively unfossiliferous deformed sediments of the greywacke suite (Torlesse Facies), metamorphosed in a high pressure environment, their lower parts to schist. The western or Hokonui zone records rapid and almost continuous marine sedimentation during the Triassic and Jurassic, with some brief local interruption during the Middle Jurassic. The eastern or Torlesse zone of thick quartzo–feldspathic greywackes and argillites, with some spilitic lavas, occupying a Pacificmargin trench, has proved difficult to interpret owing to structural complexity, lithological monotony and scarcity of fossils. The metamorphic boundaries vary in age in different places, and the known fossils show a nonrandom distribution in age and space, as if basins of rapid sedimentation migrated up and down its axis throughout Permian to Tithonian time, reacting to variable thermal gradients. The Rangitata Orogeny began with precursor movements in the Middle Jurassic but climaxed in the Lower Cretaceous. The orogeny led to the main metamorphism and folding of the geosyncline, long the East phases of granite intrusion in the foreland, and the torsion and rupture of the geosyncline along the transcurrent Alpine Fault. In the Aptian, basins within the New Zealand geosyncline received sediment still of geosynclinal facies, so that it remains uncertain whether deposition was locally continuous or whether the Cretaceous is everywhere unconformable on underlying rocks. Synorogenic breccias (pre-Campanian) locally overlie schist and foreland granites that have been dated isotopically as mid-Cretaceous, and thus point to rapid uplift and erosion. By the Macstrichtian, freshwater and marine sediments indicate peneplanation and dominantly chemical weathering. The Tasman Sea is first evident in the Cretaceous, and the Cretaceous transgression, continuing in the Tertiary, reduced the land to a changing archipelago. Early Mesozoic structures are consistent with a simple continent–ocean boundary and trench trending north-west, parallel to the Mesozoic Darwin Rise. The Cretaceous orogeny superposed north-east trending structures that have dominated later history; their northern parts face the Pacific and parallel a section of the East Pacific Rise, but their southern parts face the Tasman Sea in a kind of scissors or ‘anceps’ structure. The change from a single Pacific margin to the present anceps structure facing both the Pacific and Tasman is consistent with post-Jurassic development of the Tasman and other marginal ocean basins during the Cretaceous orogeny, which entailed considerable horizontal movements of the crust. New Zealand geology can be extrapolated onto the surrounding shallow sea-floors, underlain by subcontinental crust, using evidence from outlying islands, submarine geology, and geophysics. The speculative process of rationalizing Lower Mesozoic palaeogeography is made easier if it is assumed that the early Mesozoic arc, now sinuous, was originally straighter, and that the marginal ocean basins now interrupting its course developed during the Cretaceous deformation that led to its curvature.