Z^ has surprisingly large amplitudes considering that the ambient 

 earth's field at this location is almost entirely horizontal (H) , the 

 ratio H:Z being 10:1. This implies that the anomaly sources were either 

 formed at a time when the earth's field at this location was signifi- 

 cantly different from the ambient field, or were formed at a different 

 location and then physically moved to their present site. 



In a study of the magnetic properties of basalts from DSDP sites in 

 the western Pacific, Marshall (1978) found that the remnant magnetiza- 

 tion (Jn) of basalts from DSDP Site 63 averages 0.020 emu/cc. This is 

 a very high value, comparable to the Jn found in fresh basalts from Mid- 

 Ocean Ridge axes (Vacquier, 1972; Lowrie, 1977), and the highest found 

 in the 7 sites studied. The volume susceptibility (k) was also found to 

 be unusually high for marine basalts, averaging 0.0028 as compared to 

 the average of 0.0007 reported by Vacquier (1972). This gives a 

 Koenigsberger (Q) ratio of 21, where Q = Jn/kF, the ratio of remnant 

 to induced magnetization; F being the total magnetic intensity at the 

 sample location. This value of Q is less than half the average oceanic 

 value of 48 (Vacquier, 1972), but a remnant magnetization 21 times 

 greater than induced is still large enough to account for the large 



Z . 

 a 



The data of Marshall can also be used to compute a value called 

 "apparent susceptibility" (Vacquier, 1972) to be used in model calcula- 

 tions. Apparent susceptibility (kap) is defined as the sum of remnant 

 and induced magnetization divided by the field strength: kap = (Jn -•- 

 kF)/F. Using F = 0.38 Oe, the present value of F in the Caroline Basin, 



34 



