RESULTS OF MARITIME GRAVITY RESEARCH 



65 



pression that we have to do here with three areas, 

 where the Earth's crust has been subject to an 

 increase of density and that this is the cause of the 

 positive gravity anomaUes as well as of their sinking 

 away. Apparently this sinking has not continued 

 so far that the isostatic equilibrium has been com- 

 pletely reestablished. This view is hypothetical 



and the cause of the supposed density increase is 

 still more so. It is obvious to bring it in connection 

 with the great lateral stresses that have been as- 

 sumed in the Archipelago, but whether we can ex- 

 plain it exclusively by elastic compression of the crust 

 or whether we have to assume also changes of state 

 or temperature, seems as yet impossible to decide. 



THE GRAVITY RESULTS IN THE WEST INDIES AND ADJOINING REGIONS 



Geologically speaking, the West Indies are 

 similar to the East Indies. They are likewise 

 tectonically active and the morphological properties 

 of both regions show much resemblance: island rows 

 of mountainous character, separated by deep basins, 

 and great volcanic activity. The tectonical char- 

 acter is in both cases of the geosyncline type. 



We need not be surprised, therefore, that the 

 gravity results of the West Indies show many 

 points of resemblance to the results of the East 

 Indies. The Gulf of Mexico, the Caribbean and 

 the sea west of Cuba show positive anomalies in 

 the same way as the deep basins of the East Indies. 

 The same remarks may be made concerning them. 



North of Porto Rico, north of Haiti and near 

 Windward Passage a similar strip of strong negative 

 anomalies was found as that which has been dis- 

 cussed for the East Indies. The continuation of 

 this strip to the westward is still imccrtain. It is 

 possible that the negative values obtained in the 

 Bahamas, although less intense and not concen- 

 trated in a narrow strip, may be considered as such, 

 but the most probable interpretation of the data 

 seems to be that the critical zone continues along 

 the Bartlctt Deep and that, for this part of the 

 strip, the relative movement of the two parts of the 

 crust Is nearly parallel to it. In this way there 

 would be no compression perpendicular to the 

 direction of the trough and it might even be that 

 there is a small component of the relative movement 

 of both crustal parts away from each other. This 

 would explain the deep as a rift-formation in con- 

 trast to mo.st other deeps, which, from the point of 

 view of the buckling hypothesis, have to be con- 

 sidered as compressional features, revealing the 

 downward movement of the crust in the buckling 

 zone. This explanation of the Bartlett Deep is in 

 harmony with the irregular submarine topography, 

 suggesting sunken blocks.^ It is likewise in agree- 

 ment with the gravity field, which shows irregular 



' This agrees with the views of Taber expressed in several 

 of his publications. 



and not very intensive anomalies over the Bartlett 

 Deep. There is no clear evidence of a narrow strip 

 of negative anomalies, as has been found near all the 

 East Indian deeps and over the Nares Deep north 

 of Porto Rico. 



The gravity profile over the strip north of Porto 

 Rico is nearly identical with the profiles over the 

 strip south of Java; the horizontal dimensions as 

 well as the difference between the negative anomalies 

 in the axis of the strip and the positive anoma- 

 lies beside it are about the same. This suggests a 

 similarity of both phenomena. 



This suggestion is further confirmed by the fact 

 that the more westward profile, the gravity profile 

 over Windward Passage, shows much resemblance 

 to the profiles west of Sumatra, i.e., the negative 

 anomalies in the strip are less and the transition to 

 the positive anomalies on the Atlantic side is more 

 gradual than north of Porto Rico. In the same 

 way as for the strip west of Sumatra we have here 

 that the most {irobable direction of the relative 

 movement, E. N. E., is nearly parallel to the strip 

 and this suggests the same explanation of this 

 changed gravity jirofile: no buckling but an over- 

 riding of the northern block by the southern block 

 because of a small component of the relative move- 

 ment perpendicular to the strip. If the above 

 explanation of the Bartlett Deep is right, this 

 component gets zero and changes sign where, still 

 further westward, the strip gets again another 

 direction. 



The gravity field in the Bahamas merits a short 

 discussion. The whole area shows negative anoma- 

 lies but the deep basins are stronger negative than 

 the islands. In this regard this region is one of the 

 few exceptions to the second rule; the anomaly 

 does not increase, in a positive sense, when going 

 from shallow water to deep water but we find the 

 reverse. The most probable interpretation seems 

 to be that we have to do here with a stiff crustal 

 block that has been sinking away and on which the 

 islands have been elevated by the coral-reef-builders; 



