|
Asia and Australasia |
Sedimentology Laboratory, New Zealand Geological Survey, University of Canterbury, Christchurch, New Zealand
Subduction of the Pacific plate underneath the North Island of New Zealand began near the beginning of the Neogene, when the Tonga-Kermadec Subduction System propagated southwards into the New Zealand continental crustal block, together with the southwards migration of the relative pole of rotation between the Pacific and Australian plates. Many geological and geophysical aspects of the North Island, both on land and offshore, are in agreement with models of subduction systems from elsewhere. Several NE-trending zones can be distinguished: Taupo Volcanic Zone (volcanic arc), Axial Ranges (exposed part of arc massif), East Coast Depression (forearc basin), Coastal Ranges (structural high of subduction complex), continental shelf and slope (trough-inner-slope of subduction complex). A west-dipping Benioff zone underlies the volcanic arc and arc massif. A negative gravity anomaly coincides with the surface extension of the Benioff zone. However, no subduction trench exists below the negative gravity anomaly. Sensu stricto, The Hikurangi Trough, to the east of the subduction complex, is not a subduction trench.
The forearc basin and subduction complex are riding passively on top of the Pacific plate to the east of the Benioff zone. The forearc basin initially was shallow marine, but subsequently was filled with coarse debris shed from the rising arc massif, and is now above water. The subduction complex consists of highly deformed autochthonous Cretaceous-Palaeogene strata, possibly deposited in a pre-subduction continental borderland. Accreted oceanic sediments have not been recorded, but may be present in the lower part of the subduction complex, or, to a large degree, may have been subducted. During the formation of the subduction complex, small basins were created by elongate narrow structural ridges parallel to the trough-inner-slope. Several are now exposed in the Coastal Ranges structural high. Their sedimentary fills comprise sediment gravity flow deposits, hemipelagic muds, and arc-derived ash. Similar sedimentary facies have been cored in present-day offshore slope basins. Some sediment gravity flows reach the Hikurangi Trough via submarine canyons, bypassing the slope.
A directional, morphological and geophysical break exists between the North Island and Kermadec subduction systems, probably caused by the Vening Meinesz Fracture Zone.
Early-upper Miocene rhyolitic tuff beds in trough-inner-slope flysch basins are the first sign of volcanic arc activity. They cannot have come from the Quaternary Taupo Volcanic Zone, but were probably derived from the upper Miocene Coromandel Arc further to the NW. As subduction is oblique, strike-slip movement has moved the forearc region east of the Axial Ranges from a position closer to the Coromandel Arc to its present situation.
