SECT. 2] TKENCHES 429 



of it. These computations are based upon a fit between observed gravity, 

 seismic velocities and indicated thicknesses of layers (Fig. 8a). Whereas more 

 geophysical work might modify the minor detail in Talwani's section, there is 

 enough information to accept his mass distribution as essentially established. 

 The isostatic gravity anomahes over the trench (and the continuation along 

 the Barbados Ridge) are still in the neighborhood of —180 mgal, indicating 

 quite clearly that assumption of isostatic equilibrium here is incorrect ; in 

 other words, the required high-density antiroot under the trench to balance 

 the mass deficiency represented by the water does not exist. The degree of 

 departure from equihbrium may be estimated nicely from Talwani's data. 

 Assuming that all the mass anomaly is above some arbitrary depth such as 

 40 km, one can compute the pressure at this depth for a crustal column in the 

 middle of the trench and compare it to a standard oceanic column or to the 

 columns on either side of it in Talwani's diagram. The result of such a calcula- 

 tion shows the pressure under the trench to be somewhat greater than 1000 kg/ 

 cm 2 less than a standard column. This is the critical factor in any attempt to 

 explain the structure and dynamics of a trench. 



Drawing an analogy to the behavior of the crust when loaded by ice accumu- 

 lation gives a picture which can easily be appreciated. If a lobe of ice 120 km 

 wide, 10 km thick at the center of the lobe, and several hundred kilometers 

 long were resting on the crust in isostatic equilibrium and were instantaneously 

 removed, then the resulting gravity anomahes and mass deficiency would be 

 similar to the Puerto Rico Trench. Uplift would be rapid and equilibrium 

 would be approached in at most a few tens of thousands of years. Thus, if one 

 were to postulate formation of the trench by extension, any anomaly which de- 

 veloped would disappear rapidly ; but an even more serious objection is that there 

 is no reason why so large a departure from equilibrium should ever have come 

 about. On a much smaller scale (10 km), one can appeal to the floating cork- 

 block type of analogy. In this case various blocks are tilted, depressed or 

 raised such that the positive and negative anomalies compensate for one 

 another. The large negative one over the Puerto Rico Trench is not com- 

 pensated by positive anomalies in the adjacent areas. An extension hypo- 

 thesis of the origin of the trench cannot account for large departures from 

 isostatic equilibrium. 



Most geologists would agree to the probability that the Red Sea structure 

 represents extension. Girdler (1958) analyzes the geophysical data for this area 

 and comes to the same conclusion. The gravity anomalies are generally positive 

 and small. Similarly, most of the free-air anomalies over the Bartlett Trouo-h 

 (Cayman Trench), generally considered to have formed by extension, are 

 small, and the isostatic anomahes commonly strongly positive. The conclusion 

 must be drawn that compressional forces are acting on the rock beneath the 

 Puerto Rico Trench and other trenches sufficient to hold them down. The 

 magnitude of the vertical component of this force is in the neighborhood of 

 1000 kg/cm 2. 



It now seems necessary to construct a hypothesis which would adequately 



