| Abstract: | The East Coast Fold Belt (ECFB) of the North Island, New Zealand, is the continuation of the Tonga-Kermadec arc-trench system. Structurally its tectonic front to the east defines the Indian-Pacific plate boundary. This, however, is not continuous with the Kermadec Trench. Large-scale fragmentation of the ECFB into segments of greatly varying width, strike and structure may be caused by a strongly segmented subducting plate, individual segments of which strike in different directions and have different dips and rates of subduction. Towards the southwest, regional change of strike with respect to plate motion has resulted in the formation of a broad shear zone marked by a strongly subsiding trough filled with rapidly deposited, largely undeformed sediments in front of the ECFB. This foredeep (Hikurangi Trough), which thus occupies the gap between ECFB (Indian plate) and the continental Chatham Rise (Pacific plate) is gradually being involved in the overall deformation, due to continuing motion of the Pacific plate to the southwest, in a slightly oblique sense along the shear zone. As a result, the Hikurangi Trough is shifting with time to the east-northeast. From a tectonic, structural and morphological point of view, it is unrelated to the Kermadec Trench which terminates in the region of East Cape.The structure of the ECFB is characterized mainly by extension normal to the plate boundary, with regional tilting and down-faulting of the continental margin. Effects of compression are observed only locally, and are often due to diapiric uplifts caused by widespread, undercompacted shale. Such diapirs form elongate structural highs which in many cases have supplied sediments into adjacent basins on their landward side. Overall the continental slope and margin are underlain by land-derived sediments which exhibit in-place deformation. Locally they extend as undeformed sediment aprons beyond the tectonicfront. There is no compelling evidence of a subduction complex of imbricate thrust slices. It is concluded that the tectonic evolution is not controlled by accretion but rather by subsidence and tectonic erosion along the continental margin. The conditions are complicated, however, because of the discrete change from an oceanic arc-trench subduction system to an intercontinental shear zone. |
