RESULTS OF MARITIME GRAVITY RESEARCH 



63 



at the lower boundary of the crust of the lighter 

 sialic layer in the denser simatic layer. In case we 

 assume a second density-discontinuity in the crust 

 itself, a part of the mass-defect will occur in the 

 same way at this latter boundary. 



This explanation leads to the hypothesis that 

 the crust of the Earth of a thickness of some 25 

 km is buckling inwards along the axis of the strip 

 and that only a relatively shallow upper layer is 

 folding and overthrusting outwards and causing the 

 irregular topography of the Archipelago. This 

 hypothesis is in agreement with the Airy view of the 

 isostatic balance of mountain-chains, which assumes 

 great roots of lighter surface-material below the 

 mountains. That these roots are only coming into 

 existence along one axis in the present period, seems 

 mechanically sound, as it would be difficult to 

 understand that the crust should give way simul- 

 taneously along several lines of weakness, one behind 

 the other. 



Accepting this hypothesis, we have to realize 

 that the crust is bulging downwards in regions where 

 the temperature is higher. So we may safely 

 assume that its plasticity will gradually increase 

 and we have to expect that, at least partially, the 

 protuberance will flow away laterally along the 

 lower boundary of the crust. The melting and the 

 corresponding expansion of these masses will prob- 

 ably bring about a rising of the mountain area and 

 this agrees with the geomorphological facts: many 

 mountain-chains have shown rising in the period 

 following on the folding. 



The melting and flowing away of part of the 

 downward protuberance will also bring about a 

 decrease of the negative anomalies and a widening 

 of the strip. Besides this cause, there is another 

 reason for a decrease of the deviations of isostasy in 

 the later stages of the phenomenon, viz. the fact 

 that the lateral movement of the crust towards the 

 strip will automatically bring about a concentration 

 of the surface formations over it; this tends likewise 

 towards a reestablishment of isostasy. 



So we cannot expect older ranges to have concen- 

 trated roots of the same kind as in the Netherlands 

 East Indies and showing the same narrow strip 

 of negative anomalies. We may, however, expect 

 that isostasy has not been quite reestablished and 

 that there will still be left a remnant of these anoma- 

 lies. This appears to be the case; we find, after 

 isostatic reduction, an area of negative anomalies 

 below the Alps and a similar thing is found in the 

 United States under the old range of the Appalachi- 



ans and in other instances. In these last cases it is, 

 however, possible that the anomaly may be explained 

 by deviations of density in the crust itself. 



The Indian Archipelago shows likewise an instance 

 of a strip of smaller negative anomalies that is likely 

 to be the remnant of an older folding phenomenon, 

 i.e., a strip running over the arc of les.ser Sunda 

 Islands and continuing over the inner Banda Arc. 

 It is possible that there are more of these strips in 

 the western half of the Archipelago, but the lack 

 of gravity values on the islands prevents any cer- 

 tainty on this point. 



The geologist.s- have, however, found a strip of 

 another kind in the Archipelago which merits 

 special mention in this connection. This is a strip 

 of strong sedimentation in recent times, which 

 discloses some evidence of folding but no efifects of 

 strong lateral compression, as is found on the 

 islands in the strip of strong negative anomalies 

 (Timor, Key Islands, Ceram). This other strip 

 is found in east Sumatra, in north Java, in S. W. 

 Celebes and in east Borneo; it is not continuous but 

 it is interrupted in some places. Where gravity 

 has been determined above this strip, it shows small 

 negative anomalies or at least smaller positive ones 

 than those in the neighborhood. I think we may 

 safely interpret this strip as a down-bending of the 

 crust, without buckling, and a filling up of this 

 trough with sediments. The negative anomalies 

 may be explained by the smaller density of these 

 sediments without its being necessary to assume 

 mass-defects at the lower boundary of the crust by 

 the replacement of denser subcrustal material by 

 lighter crustal material. We may thus consider 

 this as an instance of a strip of small negative 

 anomalies, that has another meaning than a remnant 

 of an older folding phenomenon. We may conclude 

 that we have to be careful in interpreting these 

 strips of smaller negative anomalies; they may 

 point to old folding but they may likewise be ex- 

 plained by a thick layer of sediments which has 

 formed in a sunken strip of the crust. 



Getting back to the main feature of the gravity 

 field, the strip of strong negative anomalies, I may 

 mention that the root at the lower boundary of the 

 crust must have dimensions of some 25 x 50 km 

 in order to explain the magnitude of the anomalies. 

 Assuming that the crust has a thickness of 25 km, 

 the root must have a width of at least 50 km, 



2 "Het Neogeen in den Indischen Archipel," by Prof. 

 Dr. J. H. F. Umbgrove;Tydschr. Ned. Aardryksk. Genoots- 

 chap, 1932, no. 6. 



