170 



WORZEL AND HARRISON 



[chap. 9 



is a large negative-gravity anomaly. The crnstal thickness is again intermediate 

 in the region of the negative-anomaly belt and there is a big accumulation of 

 sediment. 



Raitt et al. (1955) have made seismic measurements in the Tonga Trench 



NARES 

 BASIN 



PUERTO RICO 

 TRENCH 



PUERTO 



RICO 



VENEZUELAN 

 BASIN 



KILOMETERS NORTH OF LATITUDE IB* N 

 SOO 200 100 «I3. 0'.35 



-500 



-€00 



100 



-ax> 



Fig. 28. Structure section across Puerto Rico Trench computed to fit the seismic and 

 gravity data. (After Talwani, Sutton and Worzel, 1959.) 



and in the nearby regions. More recently, gravity measurements have been 

 made as a part of the International Geophysical Year cooperation between the 

 Lamont Geological Observatory and the Bureau of Mineral Resources of 

 Australia. Fig. 30 shows the section deduced by Raitt and the computed gravity 

 anomalies for this section, using the Nafe-Drake density curve, compared with 

 the observed anomalies (Talwani, Worzel and Ewing, 1901). The comf)uted 

 anomaly curve falls at least 100 mgal below the observed points in the region 



