Finding the continental shelf – integration of geology and geophysics R Wood Institute of Geological and Nuclear Sciences, PO Box 30-368, Lower Hutt, Telephone 64-4-570 4867, Fax 64-4-570 4803, Email [email protected] Abstract Article 76 of the United Nations Convention on the Law of the Sea (UNCLOS) specifies how the limits of the continental shelf beyond the 200 M Exclusive Economic Zone (EEZ) will be determined, but it is not clear how these limits will be established along complex margins such as those that typify the New Zealand region. The seafloor around New Zealand is characterised by ridges and plateaus that reflect the complex tectonic history of the region. Rifting has resulted in crustal thinning and continental fragmentation, and subduction processes have led to continental growth by the accretion of terranes and the formation of island arcs. Analysis of geophysical data reveals the geological and morphological connection of a large continental fragment, Gilbert Seamount, with the New Zealand landmass. The saddle between the seamount and the Challenger Plateau was formed by rifting prior to seafloor spreading in the Tasman Sea. Continental prolongation to Gilbert Seamount could form the basis for extending New Zealand’s continental shelf beyond 200 M. The Hikurangi Plateau was accreted to the New Zealand landmass more than 70 Ma. The geological and morphological connection extends along the length of the Chatham Rise. Identification of the boundary between continental and oceanic rocks along the northeast margin of the plateau requires integration of all the geophysical and geological data from the region. The Hikurangi Plateau is currently being subducted beneath the North Island, but the connection of the plateau with the Chatham Rise shows that a subduction trench/plate boundary does not necessarily define foot of slope positions. The data collected as part of the Continental Shelf Project were not designed for resource assessment. However, they will contribute to clarification of responsibility for managing the resources in New Zealand’s continental shelf, and several lines show sedimentary basins of exploration interest. Introduction Analysis of these existing data included determining for each The United Nations Convention on the Law of the Sea area which aspects of Article 76 were relevant, assessing if (UNCLOS) came into force in November 1994. Article 76 of the quality of the existing data was sufficient to delineate the United Nations Convention on the Law of the Sea New Zealand’s continental shelf, and finally identifying (UNCLOS, 1983) describes conditions that allow maritime where and what kind of additional data were required. Survey states to extend their legal continental shelf beyond their lines were located to complete determination of the 2,500 m 200 nautical mile (M) Exclusive Economic Zone (EEZ). isobath and bathymetric foot of slope positions at intervals Essentially these conditions require the maritime state to of less than 60 M. They were also designed to demonstrate demonstrate prolongation of the land territory, and Article the relationship of the rocks on and beneath the sea floor to 76 outlines several ways in which this can be done. those onshore, and to find areas of significant sediment thickness beyond the foot of slope positions. New Zealand signed the United Nations Convention on the Law of the Sea and began its Continental Shelf Project in Understanding the project requires an understanding of 1996. The early phases of the project compiled and analysed Article 76, and how the complexities arising from New existing data (bathymetry, seismic, gravity, magnetics, rock Zealand’s location along an evolving plate margin affect samples) in order to minimise the acquisition of new data. the interpretation of the Article. 2002 New Zealand Petroleum Conference Proceedings • 24-27 February 2002 1 Figure 1: Satellite gravity map of the New Zealand region showing the 200 M EEZ and the location of surveys undertaken as part of the Continental Shelf Project. Article 76 sufficient to encompass all aspects of the complex geologic features that characterise active continental margins such as Within UNCLOS, Article 76 defines the continental shelf those around New Zealand. and outlines methods to delineate its outer limits where the shelf extends throughout and beyond 200 M from baselines New Zealand setting of the territorial sea. Extension of the continental shelf gives The total land area of New Zealand is about 250,000 km2. It is the Coastal State responsibility for management of the natural surrounded by large submarine plateaus, typically lying at resources on and beneath the seafloor within this region, depths of 500-2,000 m, and deep ocean basins 4,000-5,000 m including control over the exploration, exploitation, and deep. The 200 M EEZ covers an area of about 4,000,000 km2. conservation of resources. It does not include control of the resources in the water column. New Zealand has had a dynamic geologic history, strongly affected by plate tectonic events for at least the last 230 Ma. Article 76 defines the continental shelf to consist of “the As a result, the geology and bathymetry of the wider New submerged prolongation of the land mass of the coastal Zealand continental region are complex, characterised by state”, specifically excluding the deep ocean floor with its plateaus, ridges, troughs, seamounts, volcanic arcs, fracture oceanic ridges. The maximum limit of a claim cannot exceed zones and oceanic spreading centres (Figure 1). The 350 M from the coastal baselines or 100 M from the 2,500 m continent is a complex amalgamation of rifted crustal isobath, except along plateaus, rises, caps, banks, and spurs plateaus, accreted terranes and volcanic arcs. that are natural components of the margin. Within that limit, the factors governing the extent of the continental shelf Three phases of plate convergence have affected the New beyond the 200 M EEZ are continental prolongation, the Zealand region in the last 230 My. A major period of 2,500 m isobath, the foot of the continental slope, and the continental growth took place along the Gondwana margin sediment thickness beyond the foot of slope. The continental from the Triassic to the Early Cretaceous (230-115 Ma) and shelf can be extended up to 60 M beyond the foot of the these rocks form a major component of the basement rocks continental slope (the Hedberg limit), or to where the sediment of New Zealand (e.g., Mortimer & Tulloch 1996). Accretion thickness is at least 1% of the shortest distance to the base of of the Northland Allochthon, of the order of 100,000 km3, the slope. took place about 25 Ma (Isaac, et al. 1994), and oblique convergence is occurring along the modern plate boundary Article 76 uses a number of geologic and hydrographic terms east of the North Island and southwest of the South Island. to define the extent of the continental shelf, including slope, rise, natural prolongation, submarine ridges, and oceanic The New Zealand continental block has been fragmented by ridges. However, because these terms are used in a legal at least two major phases of rifting over the last 120 My. context, even this apparent clarity of terminology is not Fragmentation of Gondwana began more than 80 Ma and 2 2002 New Zealand Petroleum Conference Proceedings • 24-27 February 2002 Figure 2: Map showing the bathymetry of the Gilbert Seamount region. The location of the profile in Figure 3 is indicated. Contour interval is 250 m. resulted in widespread crustal thinning, basin formation and As mentioned above, Article 76 specifies several criteria for the separation of New Zealand from Australia and Antarctica. determining the extent of the continental shelf. These criteria The modern plate boundary began to develop through New require knowledge of the seafloor shape (2,500 m isobath, Zealand about 40 Ma. Evolution of this margin has involved foot of slope positions, morphological connection with the rifting, large-scale transform faulting, and oblique landmass) and subsurface structure (sediment thickness, subduction. Submarine plateaus surrounding New Zealand geological connection with the landmass). Determination are demonstrably geologically related to the land territory, of seafloor shape and subsurface structure relies on analysis the difference in elevation primarily reflecting distance from of swath/trackline bathymetry data, seismic reflection/ the present plate boundary. refraction data, gravity data, magnetic data, and rock samples. All of these geological and geophysical tools are essential The subduction system north of New Zealand has migrated to determine the relationship of continental fragments and east for about the last 20 My, forming a series of volcanic accreted terranes to the New Zealand landmass. arcs that extend northward from the North Island (Figure 1). These arcs are tied to geology of the North Island, manifest Continental fragments from rifted margins in onshore features such as the Miocene volcanics in Rifted margins are often characterised by a transition zone Northland and the formation of the Central Volcanic Region. between continental and oceanic crust that can contain fragments of continental rocks of varying size. Depending on New Zealand has the unique situation of lying on both sides the extent of rifting, these fragments can be integral parts of the of the modern plate boundary. This means that in a sense continental margin with little morphological or structural New Zealand is being subducted beneath or accreted to itself. expression, connected to the margin but with significant As a result, the plate boundary does not necessarily mark the morphological or structural expression, or completely termination of continental prolongation. separated from the margin by spreading oceanic crust. Continental prolongation The two major aspects of these fragments relevant to the application of Article 76 are This paper will discuss continental prolongation across New Zealand’s dynamic continental margin.