the Rangitaiki River, trending inland from Whakatane. Macpherson's 

 boundary faults are shown in Fig. 1. Geophysical work by Modriniak 

 (1945) indicates that sediment fills the graben to a depth of at least 

 700 ft. It may be safely inferred that the White Island Trench is the 

 seaward continuation of the \Vhakatane Graben. 



Available data do not permit detailed mapping of the sea bottom far 

 beyond White Island. Nevertheless, what soundings there are (Fig. 2) 

 suggest that the White Island trench has considerable extension to the 

 north-north-east towards the Kermadec Islands. The soundings shown 

 in Fig. 2 are too widely spaced to allow determination of the width of the 

 trench over most of its length. The bottom of the trench remains at 

 approximately 3,000 ft. below the adjacent sea-floor, and its greatest 

 known depth below sea-level, 2,050 fathoms, at a point 140 miles south- 

 west of Esperance Rock, constitutes the deepest part of the sea bottom 

 between the East Cape-Kermadec Ridge to the east, and the Gazelle 

 Basin to the west. The low ridge west of the trench suggests a structural 

 " high " which has no precise analogy in the known structure of the Bay 

 of Plenty area. 



The relationships between the White Island Trench and other known 

 structural lineaments are fairly obvious. It is parallel to the greater 

 submarine welt and foredeep of the East Cape-Kermadec Ridge and 

 Kermadec-Tonga Trough, and occupies the position of a " back-deep " 

 on the inner side of these folds. It is homologous with the complex 

 volcano-tectonic depression of the North Island of New Zealand to which 

 the name Taupo Graben is applied, and further work will probably show 

 continuity. Still more clear is the relation of the submarine graben to 

 the line of active and dormant volcanoes which Dieffenbach in 1843 

 recognized as " one connected hearth of volcanic action." 



References 



Macpherson, E. O. (1944) : N.Z. foiirn. Sci. &^ Tech., Vol. 26, No. 2, (Sec. B), 



pp. 66-76. 

 Modriniak, N. (1945) : N.Z. Journ. Sci. &- Tech., Vol. 26, No. 6, (Sec. B.), 



pp. 327-331. 



Summary of Discussion 

 Dr. Gutenberg emphasized that such ridges separated by troughs or 

 grabens occurred very widely, and were not an isolated phenomenon. 

 He instanced the island ridges of the East Indies separated b}' troughs. 

 Other such regions were quite frequent in the Pacific, although their 

 significance was rather obscure. 



Mr. Fleming agreed with the last speaker. He pointed out that the 

 White Island trench was a collapse rather than a downfold, but agreed 

 that as such it is by no means unprecedented. 



Dr. Kuenen maintained that the present example was important as 

 illustrating a trench actually continuing into the continental shelf as a 

 graben. Dr. Gutenberg considered that a gravity profile across such a 

 ridge and trench would be very valuable, and Mr. Fleming indicated that 

 a gravity meter was to be brought into use, if possible, this year. 



212 



