64 



INTERNATIONAL ASPECTS OF OCEANOGRAPHY 



according to the buckling hypothesis, and we get a 

 shortening of the crust of about 50 km. This 

 figure is confirmed by other considerations. 



This confirmation has to do with the gravity 

 anomahes found west of Sumatra and east of the 

 Philippines. Here the strip has no longer the same 

 symmetric character that it has in the middle part 

 of its course through the Archipelago; the negative 

 anomalies are smaller and the transition to the 

 positive values on the ocean side is less sudden. 

 Taking into account the character of these gravity 

 profiles and the fact that in these parts the direction 

 of the strip is nearly parallel to the most probable 

 direction of the stress, S. S. E., the most likely 

 interpretation of the phenomenon seems here to be 

 that Sumatra and the Philippines are sliding along 

 the line of the stri;) combined with a shght over- 

 riding of the ocean-floor. This would mean that 

 the movement is nearly parallel to the strip with 

 only a small component perpendicular to it. The 

 downward bending of the ocean-floor would bring 

 about a gravity profile of the character that is 

 mentioned above. 



South of the Philippines, near the Talaud Islands, 

 and .south of Java, the direction of the strip changes 

 and makes a greater angle with the direction of the 

 stress; the gravity profile assumes at once another 

 character that points to a buckling of the crust in 

 the way we have discussed it. We found a figure 

 for the shortening of the crust, corresponding to this 

 buckling, of about 50 km. Now it is a remarkable 

 coincidence that the geologists are inclined to think 

 that Java has moved southward with regard to 

 Sumatra and that the amount of this movement is 

 estimated at about 40 km. It is likewise remarkable 

 that the bathymetric curves south of the Philippines 

 show a similar bulging towards the east of the same 

 amount; this topography is now known in detail 

 thanks to the expedition of the Snellius under the 

 leadership of Van Riel which has made more than 

 30,000 soundings in the Eastern part of the Archi- 

 pelago. 



These facts provide us with a welcome confirma- 

 tion of the buckling hypothesis; they point at least 

 towards a connection of the gravity field with 

 horizontal movements of the crust. Another still 

 stronger confirmation is given by the fact that 

 the only islands where great overthrusting has been 

 stated since the beginning of the Tertiary period^, 

 are those islands that are located over the .strip: 

 Timor, Tenimber Islands, Key Islands, Ceram, 

 Buru, and the eastern part of Celebes; the other 

 islands over the strip are not yet known sufficiently. 



These overthrustings date from the earliest part 

 of the Miocene, which puts them about 8,000,000 

 years back. 



It is a remarkable fact that these islands have not 

 experienced great deformations in the more recent 

 periods. Still it seems probable that the great 

 tectonic phenomenon is continuing in the present 

 period, because the earthquakes are violent and 

 frequent and the localization of their centers indi- 

 cates that it is still the same strip that is active. 

 These facts suggest the continuation of the lateral 

 compression of the crust without the taking part 

 of the surface layer. It makes the impression as if 

 the crust is pushing together under the islands and 

 disappearing downwards, while the islands have 

 been elevated above the region of compression, so 

 that they are not partaking of the great deformation 

 save some block-faulting movements that have been 

 going on on some of these islands; this block-faulting 

 proves that the apparent quiescence is not real 

 and that greater phenomena are going on in deeper 

 layers. A comparison of this hypothesis with 

 what is known about the tectonic history of the 

 Alps gives promising outlooks for the understanding 

 of the mechanism of mountain-formation; I may 

 refer here to another paper about this subject.' 



Lastly I wish to mention a remarkable correlation 

 of the course of the strip of strong negative anomalies 

 with the distribution of volcanoes in the Archipelago. 

 Nearly every curve of the strip is accompanied on 

 the inner side by a parallel row of volcanoes at a 

 distance of a few hundred kilometers. This corre- 

 lation appears to be in harmony with the buckling 

 hypothesis, which makes it understandable that the 

 crust, when moving towards the strip from the inner 

 side of a curved part, is subject to tensile stresses 

 in a sense parallel to the strip. That this circum- 

 stance will facilitate the formation of volcanoes, 

 seems acceptable. 



Examining the fields of positive anomalies in the 

 Archipelago, we see that there are three fields of 

 especially strong anomalies and these fields coincide 

 with three deep basins, the Celebes Sea, the N. W. 

 Banda Sea, south of the Sula Islands, and the Banda 

 Sea, west of the inner Banda Arc. With the 

 exception of the typical throughs, all other parts of 

 the seas in the Archipelago are less deep. These 

 basins show all the same morphological type, a 

 smooth and even bottom and steep sides. 



Taking the evidence together, we get the im- 



^ The Mechanism of Mountain-Formation in Geosyn- 

 clinal Belts, by F. A. Vening Meinesz, Proc. Amsterdam 

 Acad. Sc. Vol. XXXVI, No. 4, 1933. 



