SKCT. 1] 



GRAVITY AT SEA 



169 



physical investigations have been made by Vening Meinesz and others to try 

 to dehneate the structure related to these belts. 



In the East Indies, Vening Meinesz has evolved a detailed and complicated 

 theory to account for the anomalies. In summary he invokes a compression 

 which causes a plastic downbuckle of the earth's crust causing the earthquakes, 

 negative gravity anomalies, and shear weaknesses, which provide lanes for 

 vulcanism in the island arcs, and uplift of the islands. The reader is referred to 

 chapter IOC of Heiskanen and Vening Meinesz (1958) for a comprehensive 

 treatment of this subject. 



Tsuboi (1957) has calculated a mass distribution for the East Indies to 

 account for the gravity anomalies. Fig. 27 is adapted from his work in which he 



Fig. 27. Structure section through the Dutch East Indies. (Adapted from Tsuboi, 1957.) 



used a density differential of 0.4 g/cm^ at the M-discontinuity. It is readily 

 seen that the thinnest part of the crust is beneath the trench. 



Probably the most detailed investigations have been made of the Puerto 

 Rico Trench. The latest work is reported by Taiwan! et al. (1959) and the reader 

 is referred to it if he desires references to the earlier work. Fig. 28 shows the 

 mass distribution computed to fit the detailed anomaly curve. Many seismic 

 data are available to control the mass distribution of the upper crustal layers. 

 The densities are indicated and were chosen to fit the Nafe-Drake velocity- 

 density curve. The depths of the layers determined seismically are indicated by 

 the circled points. The M-discontinuity was chosen to obtain the best fit of the 

 gravity data using a depth to the mantle of 11 km at a point 310 km north of 

 18°N. Here, in the trench, the crust is intermediate in thickness between an 

 oceanic and a continental crust. The free-air anomaly of -346 mgal measured 

 here is the largest negative anomaly yet found. 



Recently a continuous gravity profile has been observed with the Graf Sea 

 Gravimeter from well out into the Atlantic Ocean to well inside the Caribbean 

 Sea, passing close to Barbados. Many seismic refraction stations were available 

 from the work of Officer et al. (1959) to provide density estimates from the 

 Nafe-Drake curve and depths of the upper layers for control of the computa- 

 tions. The mass distribution required to fit the gravity anomaly is shown in 

 Fig. 29 (Sutton, Talwani and Worzel, 1960). Here there is no trench, but there 



