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A proton magnetometer was operated during the passages between the DSDP Leg 20 drilling sites. The track of D.V. Glomar Challenger (Figure 1) ran from the outer ridge southeast of Japan past Marcus Island, Guam, and Kusaie and terminated in Suva, Fiji. Residual magnetic anomalies (in gammas), derived from the shipboard readings using the IGRF coefficients of Cain et al. (1968), are presented together with the bathymetric profile at the end of this chapter. Depths are indicated in tau, the conventional echo sounding unit (1T = 1/400 sec total travel time). Along the top of the profile the time of day is marked, and along the bottom important positions are indicated. A detailed list of navigation coordinates is given in the chapter describing the seismic reflection profiles (Chapter 28). The profiles between the northern drilling sites (194 to 198) lie to the south of a group of well-defined eastnortheast magnetic lineations which were mapped some years ago by Uyeda and others (Uyeda et al., 1967; Uyeda and Vacquier, 1968). To the east and northeast of the track between Sites 198 and 202 is situated another group of linear magnetic anomalies trending northeast (the Hawaiian lineations, Figure 2). South of Site 202 the track passes over a complex area including the Solomon Plateau and the Fiji Plateau which lie to the west and southwest of some east-northeast lineations south of the Magellan Plateau. The latter (the Phoenix group of anomalies) have recently been discussed by Larson and Chase (1972) who believe that the three sets form a single series of Mesozoic lineations. In the area of the drilling sites the trends of the magnetic anomalies, if they are indeed linear, have not yet been elucidated. On the maps of Uyeda and others (Uyeda et al., 1967; Uyeda and Vacquier, 1968) the Japanese lineations reach no further south than 35°N, and on the recent map of Larson and Chase (1972) the Hawaiian group extends no further west than 160°E. If linear anomalies do exist between Sites 194 and 199, then they could belong to either the Japanese or the Hawaiian set, or both groups may be present in a magnetic bight similar to that described by Elvers et al. (1967) in the northeast Pacific. Because basement was not dated on Leg 20, the age gradients remained undetermined. It can be seen on the profile that anomalies of fairly large amplitude occur between Site 195 and a point just north of Site 198. Because much of this track runs approximately southeast, this suggests that if lineations are present they must trend in a direction closer to northeast than northwest. It appears, therefore, that the pattern in the area may be part of the Japanese set rather than the Hawaiian. Support for this conclusion comes from a consideration of the anomalies seen around Site 196 where a series of course changes was necessary before the final drilling location was established. On the approach to the site made along courses of 100°-115° on September 29, three anomalies labeled A, B, and C are seen (Figure 3). On the track from Site 197 to Site 198 (130°; October 8) these three anomalies can again be identified and indicate a lineation trend of 045°. It is interesting to note that there is topographic grain in this direction in the region of Sites 196 and 197 (Figure 3). This grain is interpreted as being imparted at the accretion center that generated the crust in this area. At Site 198 the trend of the magnetic anomalies cannot be identified from the Challenger magnetic data. However, just north of this site a prominent northeasterly topographic grain is present (Figure 4) which suggests that magnetic lineations trend in this direction also. A low ridge running northwest is seen about 15 miles to the southeast of Site 198 (Figure 5), but south of there no trend is discernible. In the latter area the amplitude of the magnetic anomalies is small (less than 80-100 gammas). By combining the magnetic and topographic data, it is suggested that the Japanese lineations reach 25°30'N which is some 1000 km further south than they had been mapped previously by Uyeda and others (Uyeda et al., 1967; Uyeda and Vacquier, 1968). By accepting the conclusion of Larson and Chase (1972) that the Japanese lineations become younger as the trench is approached, it is implied that the crust is appreciably older at Site 198 than at Site 194. Presumably the crust at Site 199 is older still, although this cannot be supported by the magnetic data since the anomalies are of low amplitude in the area. Extrapolating either from the Japanese or the Hawaiian set indicates that the crust is oldest in the area of Site 199 which is at least consistent with the thick sedimentary succession there.

Total Field Magnetic Anomalies and Topography along a Traverse Between Japan and the Fiji Islands